The brother

I feel like there’s more to their relationship. She keeps saying she did or did not do something, maybe she did something to him and he’s still holding a grudge.

While i understand why this frustrates you, he's a bullied teenager drowning his problems in videogames. Even if he says he wishes his sister was dead, he obviously doesn't mean it. What he needs is attention and an actually good psychologist. He's a very realistic representation of certain teen's going through stuff.

Let the story unfold, I feel he's a victim and the culprit was the sister, I mean her childhood is very shady