Also, I find it strange that we're even comparing the two of them. The only thing they have in common is having a sister and being a main protagonist. This is a formula that was done before even before Ivypool and Dovewing were born, with Leafpool and Squirrelflight.
Ivypool: She and her sister had made a childhood promise to do everything together, but as time went by, Dovewing couldn't keep that promise. Not only that but other cats were also teasing Ivypool for her inferiority compared to her sister's accomplishments, an example being Blossomfall. She was constantly living in her sister's shadow and always compared to her.
Violetshine: Wasn't even raised with her sister, and had a harder understanding of what actual familial relationships should be like. She went about her kithood thinking Pinenose's cold shoulder toward her was normal and that the only real cat she could turn to was Needletail. Who was the constant in her life, even if it was unhealthy. She always felt that she didn't belong and that no one wanted her in the Clan.
Ivypool: Her sister was lying to her, which caused an even bigger gap in their relationship, they couldn't click with one another anymore like they used to. Yes, Jay and Lion were making Dovewing lie to her, but they're both hypocrites, especially Lionblaze considering he tells Cinderheart later just to try and get with her. Meanwhile, Ivypool became an easy target for Hawkfrost. The first time they meet isn't even in the Dark Forest, it's in a meadow, and he offers her training techniques, to which she immediately sees improvement in herself. He manipulated her into believing that she's just as good as her sister and that she was special too, and that she matters as well. The irony is that Ivypool was excited to tell her sister about meeting him until she got back and Dovewing broke their promise again and left to go to Lion and Jay. She doesn't know about the secret at that point, so of course, being young, she misunderstands the situation.
Violetshine: Because she looked to Needletail as her only real family, her situation was more complicating. Needletail came off as manipulative to the readers, but to Violetshine she was everything. When Rowanstar let them go so easily she felt that all of her fears and doubts were right, that they really didn't want her. And that the only one that actually did was Needletail. And despite being sisters with Twigbranch, she never felt a real connection to her, it makes sense, again they weren't raised together. She is a product of her environment and ended up looking up to Needletail for everything, especially when they both join the kin. It gets to the point where she believes Needletail's ideologies and thinks everything she's doing is right. Because she has no real attachment to her sister emotionally, Twigbranch is distraught and still tries to create an emotional bond with her sister over their missing mother. This, however, doesn't work because Violetshine didn't feel the same way, and this created a gap between their relationship.
Ivypool: Ivypool was fed all types of ideologies, thinking that what she was doing would make her a better warrior. So she wouldn't have to live in her sister's shadow forever. She wanted to be stronger and better, so she could serve her clan, so she could be someone that others are proud of, and not just Dovewing's inadequate sister. However, she starts to realize that things aren't how they seem. She wasn't alive when certain cats like Brokenstar, Tigerstar, Hawkfrost, and Mapleshade were, so she didn't know their cruelty first hand. She honestly believed they had changed their ways, and they made her believe that. But eventually she found out the truth, and when she tries to escape she's almost drowned.
Violetshine: On the other hand, Violetshine's reality check was when she saw just how cruel the kin could be. When Rain tries to attack Darktail so he could take over as leader, Darktail severely punishes him, taking one of his eyes. And forcing him to submit in front of everyone, including Needletail. This, understandably horrifies Needletail, but more so Violetshine, who like Ivypool, was still at a very young and impressionable age. Violetshine didn't feel that this was a place she should stay, otherwise, she'd end up like Rain, or worse.
Ivypool: She tells her sister the truth, and they tell Jay and Lion. She vows that she never wants to go back again, because she learned her lesson, and realized the truth. Unfortunately, it's not as easy, every time she sleeps she goes to the Dark Forest, she can't control it, just like the many trainees that were also swayed and manipulated. The only escape was death. Jay and Lion then both force her to become a spy, something she didn't want to do, and when she refuses, Dove convinces her to do it, and also finally tells her about her powers. Ivypool becomes sympathetic with her, they finally make up and try to work through their troubles together. Ivypool agrees to become a spy, although she didn't have much choice in the matter.
Violetshine: Escapes to rejoin ShadowClan, unlike Ivypool, she had a choice, it's not the same in the waking world as it is in the dream world. Violetshine isn't swayed by her clan's suspicions of her and is determined to prove that she is loyal. She and her sister seem to start reconnecting, slowly but surely. However Needletail shows up with Rain and they intimidate and threaten her, angry with her betrayal to them and the kin, Needletail seems to take it more personally. Whereas she felt abandoned by Twigbranch when she left to rejoin ThunderClan, she now feels like she had abandoned Needletail and feels fear and guilt.
Ivypool: Ivypool is now a spy, her objective is reconnaissance, to gather information about the Dark Forest. However it's not that easy, Mapleshade literally tells her as long as she's still alive they won't trust her. She goes out of her way to fight and prove her loyalty to them, killing Antpelt twice, even almost attacking and killing Flametail at one point. She slyly tries to ask about their plans, and what their motives are several times, but they're too careful around her. The one time she actually has a chance to find out their tactics is when they held a meeting, she requested to join, but they denied her. And then after the meeting, she snuck in to look at the plans written in the dirt. However, Dovewing woke her up before she could see them, ruining one of her chances of finding out their plans. Later, like the others, it is revealed to all of them, what their true motives and plans are, which wasn't until the last second, not long before the war itself. Beetlewhisker spoke against their motives and is killed for it. Brokenstar uses a fear tactic to keep them in line, while Ivypool had to still seem loyal to them by kicking Beetle's corpse, and acting like a DF cat so she didn't end up the same way. As soon as she found out their plans she reported back to Jay and Lion, along with Firestar.
The only thing that Ivypool didn't tell them about was WHO was training in the Dark Forest. She didn't want these cats, who were manipulated and mislead just like her initially to be judged for their actions and punished. Dovewing, who is capable of hearing all the way into the DF, and would have been a more appropriate spy by circumstances, says the same thing and refuses to tell who the cats training there are. Blackstar, and Lionblaze (hypocrite) are so quick to want to punish and possibly kill these cats who are just as scared and confused as the next cat. It is only when Brambleclaw reassures Ivypool they would not be punished and/or killed, that Ivypool tells him who are trainees in ThunderClan. Later when the time comes, it is Ivypool that convinces these same cats to fight for their clan and help change the tides of war.
Violetshine: When the rogues come to take over the clan, Violetshine initially was going to leave with Rowan, Tawny, and Tiger, but stayed because of Needletail. Needletail emotionally manipulated her into staying, similar to what Dawnpelt's kits did. During her time with the kin Violetshine is put into wars and even forced to attack her own sister at one point, she feels utterly guilty about possibly crippling her sister too. But she was pressured into doing so, like Ivypool, she was young and impressionable, and didn't know who else to turn to but Needletail.
When Rain is killed, Needletail finally has her reality check and realizes that her ideologies for a better clan life were different from Darktails. But unlike Violetshine, who realized it sooner, she realizes it too late. Needletail starts having a change of heart, even helping the other clans with herbs at one point, and helping the prisoners. Violetshine becomes a spy, but her objective is a rescue mission. She keeps the other clan cats informed, which is different from Ivypool's mission. Her mission was to tell them of the updated status and events of what was happening with the kin, which was more or less closed off to the other clans. While Ivypool had to tell Jay and Lion about the events going on, nothing more, however, she doesn't find out about the plans of the DF cats until the last minute like the others. And unlike the trainees which were in every clan, and had ears and eyes in every clan, the other clans only had Violetshine as a real source of information.
Ivypool: Ivypool is able to keep up her guise long enough until she's blatantly caught betraying the Dark Forest. She reveals that she was never loyal to them, which is true and fights against them. She's immediately attacked by three cats and is almost killed for her betrayal until Hollyleaf jumps in and saves her. But dies in doing so.
Violetshine: She's tasked to knock out Darktial and his crew with seeds to put them to sleep, but her plan fails and she's dragged away to be punished. However Needletail is involved, they force her to either kill Violetshine or they would kill them both. Needletail instead chooses to save Violeshine, sacrificing herself so the other could escape to the clans, while Needletail is killed by her once clanmates and Darktail and his group.
Ivypool: Ivypool was never truly loyal to the Dark Forest. She was manipulated thinking that she would become a better warrior with their help. She truly believed in this, but she wasn't loyal to them.
Violetshine: She was loyal to Needletail in the end, but mostly felt that she didn't belong. She had an unhealthy emotional attachment to Needletail, and because of her sacrifice, in the end, she still holds her to a good light.
Ivypool: Her issues with her sister were solved when she was told the truth about Dovewing's powers, and Ivypool told the truth about her being in the Dark Forest. This is where their "rivalry" ended, and Ivy and Dove became more understanding and forgiving of one another.
Violetshine: Her issues with her sister become solved after the kin are defeated, she and SkyClan return. She starts to believe that they can finally start over and that she has somewhere where she belongs, with her real family in SkyClan. She starts to realize that what she didn't think she needed this whole time was actually what she wanted all along, a real family.
However whereas Ivypool and Dovewing's intial problems are solved, Twigbranch's and Violetshine's continue. While Ivypool and Dovewing have new issues...
Ivypool: While she's in the Dark Forest she finds out that Tigerheart is training there, she is suspicious of him. And doesn't trust him, makes sense since, until the very end of the war, his motives and reasoning were a shroud to the readers and mostly Ivypool. Ivypool talks to her sister about it, insisting she shouldn't see him anymore, she obviously cares for her sister and didn't want her to get hurt. Thinking that Tigerheart is just using her since he's a trainee in the Dark Forest. Dovewing refuses to believe her and uses her being in the Dark Forest as a jab of her being unloyal when Dovewing ironically was seeing a tom from another clan and breaking the code knowingly. Eventually, their relationship even led to Ivypool being captured, and used as exchange bait for herbs, thanks to Tigerheart betraying Dovewing's trust, and using her naievty and information to get what they needed for their clan. (On top of that Dovewing literally runs back to the clan and pretends to sleep and doesn't say a peep while the others figure out what happen on their own and Ivypool is punished for Dovewing's actions).
I mostly don't hold any of this to Dovewing though, like Ivypool, she was also young, and impressionable. However it's when they get older does it become a problem, and it shows that Dovewing hasn't matured. Whereas Ivypool got over her jealousy over her sister when they were still young apprentices, and they forgave one another, Dovewing still continued to be naive and had an on and off relationship with Tigerheart. When Dovewing chooses Bumblestripe, she's protective of her sister's business when Tigerheart asks about her. When she also supported her sister being with Bumblestripe, which makes sense since she and Bumble are good friends, and he even confides in her about Dovewing. When they break up, Ivypool doesn't hold anything against Dovewing, the two of them are still close and supportive of one another. However, it's when she finds out that Dovewing is seeing Tigerheart, yet AGAIN, does the gap in their relationship return. Ivypool, obviously, doesn't support her sister breaking the code, while she herself remains loyal to her clan. She tells her sister that she should stop, they're adults at this point, she should absolutely know better, and if Dovewing is so desperate for a mate she should look for one in their clan. She doesn't coddle Dovewing anymore at this point, she refuses to support her breaking the code and basically reassuring her that what she's doing is alright. Yet despite all of this she STILL keeps Dovewing's secret, because she cares about her sister.
When Dovewing goes missing, she's distraught, and out of all the cats, she's on patrols looking for her. Whens she finds out that Tigerheart just let her go she strikes him out of anger, that he'd let her sister go off alone into the dangerous unknown, knowingly. It would have been worse had she known Dovewing was pregnant, something neither he or Dove told her. When her sister finally returns she's understandably upset, because Dovewing had left without saying anything, worrying everyone, and when she finally returns she has Tigerstar's kits and was now living in ShadowClan. Ivypool eventually makes up with Dovewing, unlike most of the other cats that branded her a traitor. But Ivypool and Dovewing were sisters and they obviously cared about one another.
Violetshine: On the other hand, with Violetshine things were more complicating. Their initial issues were still a thing. Twigbranch and Violetshine were finally reconnecting as sisters, something they struggled to do when they were younger. It makes sense though, they were in separate clans growing up, raised in two completely different environments. Whereas Twigbranch had a warm home and loving cats around her, Violetshine suffered from being tossed around constantly and living around brutish cats, without real order, she thought it was normal. But to Twigbranch, who got to see it first hand while being held in exchange for herbs, was horrified by their behavior. To her it wasn't normal, yes the two clans have different cultures, but abuse isn't normal and nor should it be normalized.
Violetshine, once she finally had a true family, felt happy and sated in SkyClan, she finally felt like she belonged for once. And although she thought of Needletail as a hero, which makes sense, she was a prominent figure in her life, and she did save her, it doesn't change that she might have been suffering from Stockholm Syndrome at this point in the story. Violeshine wanted to learn more about her kin in SkyClan, which was vastly different than in ShadowClan, and Darktail's group. She started to realize what a real family is, what real bonds should be like, and she became loyal to her new clan.
When it comes to her relationship with Tree, in a way she saw herself in him, Tree always felt like an outsider looking in, he didn't believe he was cut for clan life. He didn't believe he belonged there, just like how Violetshine felt at one point. The two of them were able to find attraction toward one another, but it also showed that Violetshine may have been suffering from abandonment issues. It makes sense, again she was a product of her environment. She felt that her sister abandoned her several times, she felt that she abandoned Needletail at one point, she felt that her clan had abandoned her, she just felt like she was alone. So when Tree was leaving, she was panicked and desperate to help him find a reason to stay, and eventually succeeding. This issue with her mentality can also link to Twigbranch leaving her yet again when Twigbranch and Finleap leave to join ThunderClan. Twigbranch may have not felt happy in SkyClan, and that she realized the opposite of what her sister did, that kin isn't just about blood, but who you're with, it doesn't change that she was a very fickle character. Thankfully she does mature past this eventually by the end of the series.
When Violetshine was choosing to leave with SkyClan, Twigbranch confronts her, claiming she was leaving her again, but this was a good moment and a clear point between their relationship. Because Violetshine points out Twigbranch's actions up until that point, making Twigbranch realize she had done the same thing to her sister too. Because of this there will always be a wedge between the closeness of their relationship, they love one another, but they acknowledge that each of them has different wants in life, and what they desire familial wise. Twigbranch starts to mature at this point, making up with her apprentice, standing her ground against Finleap and coming to an understanding with her sister. She later brings her and the rest of SkyClan back to the lake, ironically, letting them know that they are wanted there, something that Violetshine had wanted for a long time from her sister and others.
In the end...
Ivypool: She is now a well-respected cat in her clan, and very loyal. She's trained two apprentices and has matured since her apprentice years herself. She's come to accept her sister's choices but still loves her. She now has her own family with Fernsong, who is supportive and kind, and her three kits in ThunderClan. She's also a good candidate to become leader, most likely, due to her maturity and leadership skills she's displayed over the years. But at the same time, she seems content with her current life and status, as long as she's happy.
Violetshine: On the other hand, is now settled by the lake in SkyClan, her sister has come to understand her choices, and respect them. She's become a very emotionally strong and independent character, even if she's mates with Tree now. She had chosen to leave to be with her clan over him, and he respected that because he wanted her to be happy. Now they're reunited and expecting kits, she can have a family of her own, and have them raised as the next generation of SkyClan cats, to help continue their legacy.
Both of them are different cats, with different circumstances and development.P.S. They don't look the same either.
Ivypool: a slender, soft-furred, pale, silver-and-white tabby she-cat. She has white paws, a white belly and face, white splotches covering her pelt, a striped silver tail, sliced ears, and dark blue eyes.
Dovewing: a pale gray she-cat with green eyes.
Violetshine: a small white she-cat with black splotches and amber eyes. She has glossy, short fur, and wide paws.
Twigbranch: is a gray she-cat with green eyes. She has short, fluffy, glossy fur, and her ear-tip is sliced.