I mean... okay, the brother drowned while he was there watching, but what was he supposed to do? He's like... eight or nine years old there and it looks like a flash flood. In that flashback there he's reaching out which obviously wasn't enough, but what was he going to do? Jump in and drown too?
Is she doing this perverse torture as a way to get back at him then? Or did she end up liking the fact that her older brother died for whatever reason and is now a yandere? This is one strange as hell turn.