I didn't see their parents treating them differently really. He decided to do something nice for Kayla and calls the Mom for the cookie recipe. The whole thing after the fire rescue was a clusterfuck, especially how badly it affected him. I'm not even sure if his parents are aware of the full extent of it really.
I don't see Kayla as an abuser, more of a fractured sibling relationship that resulted in them holding a grudge and bitching at each other. The MC is quick to believe that his twin sister is some kind of whore who sleeps around, when we see that's far from the truth. In the words of Emily..
so their lack of any real inquiry as to the fire doesnt stand out to you?
assuming the parents were dumb and had no clue about long term effects of smoke inhalation, they still let it pass as inconsequential. i mean not even a hug and pat on the back like im so proud of you.
but ill go back and look but the 2 times i played through it was set aback at how little interest they had.
as for his sis being a slut, we can break it down. why did those guys attack and set MC up, i mean really. they wanted in her panties, he was in the way. MC understaood what those guys were trying to do and was tyring to protect his sister. she sided with them and cut him out of his life treating him like a monster. didnt even try to give him the benefit of the doubt.
at that point why wouldnt he assume she does and goes along with everything those guys want from her. add that from that moment on everything he knows about her is rumor and 2nd hand. and her behavior, forcing him out of the house to have her current boyfriend over, what is she going to do that he cant even be in the same house for it.
as for abuse, she goes out of her way to put him down, threatens and name calls him. its abuse. and due to being found at fault for the fight and everything, she knows he cant fight back in any meaningful way. she completely treats him like trash and walks all over him.
this isnt some bickering between siblings over the remote control in a 90s sitcom.