
This is kinda making me uncomfortable... the way they act torwards their siblings rubs me the wrong way

It's also suppose to convey how mentally damaged they are in terms of dealing with affection. As Roxanne pointed out, Dion is different from Fontaine. His feelings for her aren't sexual. In the novel it's more clear that it is familial love (it's Roxanne's love for her brother Achille which is what sparked Dion's obsession for her...it's the reason why he wants to see her cry again and why even though she's insulting him about how disgusting she finds him he takes it, because it all springs from her love for her brother which is what draws him to her), but their family is so fucked up that the conception of familial love doesn't really exist as a concept that most of them can process (again, why Roxanne's ability to fascinated Dion) and so those feelings end up distorted and twisted because they end up overlapped with the only adjacent feelings they ARE equipped to process which are things like appetite and obsession and aggression.
I would fucking kill everyone in that school.