User blog comment:Shadewing/Who is the fourth apprentice?/@comment-4163707-20110719191236/@comment-1703139-20110720003342

Birchfall is Ferncloud and Dustpelt's kit. Foxleap is their son from a later litter. Birchfall is Dovewing and Ivypool's Dad, so....yeah.

When you think about it, really everyone in ThunderClan is related to ThunderClan (not by blood, maybe, but still.)