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It's because one of Judas' parents married one of Tara's mom's parents. So they were step-siblings. Then some other shit happened that can't remember that he decided not to be that bitches family and kinda cut ties with her. (Forgot which chapter it was.
He has also stated multiple times since the beginning that he isn't her uncle

Wait is she foreshadowing the fact that she might end up with her uncle???