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The end of Raphael #1, where Raphael explains to Casey that until they know who's targeting the turtles, he has to stick close to his brothers. When a disconsolate Casey tries to put on a brave face and say goodbye, Raph will have none of it: Casey is one of his brothers.
In issue #14, when Splinter calls a family meeting of the Hamato Clan, Casey and April think it's best if they leave. Splinter reminds them that it's a family meeting and asks them to please stay.◊ No doubt about it, Splinter and the Turtles see April and Casey as part of their family.