Good point too, but you guys are just assuming that she has to be Mom's kid through a cheating moment or something.
Really she could just as well be from Dad's extra marital affair, having her from a whore that he cheated with, that didn't want the child and forcing Dad to raise her after the bitch leaves his ass.
It would explain why Mom no longer gives a shit about Dad and pursues her real lust/love which is you...….and also explains why she treats her so harshly, although her ignoring her schoolwork for so long didn't help either.
Clearly some possibilities, but I think it's just a case of having variety in game, more than anything else....Lol!!………….Peace!!