Ooh, boy. There's a lot.
Goldenflower's first mate being Patchpelt and having another kit with him (aside from Swiftpaw) named Lynxkit.
Dawncloud and Finchflight being the parents of Blossomkit and Swampkit.
Whitestorm being the father of Ashfur and Ferncloud, with their littermates being named Elderkit and Tulipkit.
Snowkit having a sibling named Mistlekit. Also, Smallear being the father of both of Speckletail's litters I guess. That was proposed by Vicky and on the family tree of the old website, but never confirmed as canon before now.
Gorsepaw's siblings being Quailkit and Storkkit, with their father being Cloudrunner.
Crowfeather's siblings being Hillkit and Downkit.
Graypool and Rippleclaw third kit being named Swankit (this kit didn't even exist in CP, but was around in the FOS prologue when Graypool adopted Misty and Stone).
Mosspelt and Frogleap being the parents of Dawnflower was originally from Su's missing kits, but that got made officially canon in LH. Same with Blackclaw being the father of Mistystar's kits.
Dappletail and Runningwind being the parents of Featherkit and Cricketkit. ADP said that Dapple had a deaf white son who is Featherkit over here, but this is the first time we got a name for her two kits and a father for them.
Ashheart and Cinderfur being the parents of Turtlekit, Quietkit and Rubblekit.
Fuzzypelt and Robinwing having another litter in Cherrypaw and Chestnutkit.
Cloudtail's father being named Oliver and his siblings being named Zack, Taylor, Nami and Livy.
Darkflower and Scorchwind being the parents of Cedarheart, Rowanclaw and Lavenderkit.
Firestar's other siblings being named Luna, Filou and Tommy.