Worry about Naoya in a similar way, huh? No, you silly lesbian girl, he's only deeply envious because his friend gets to live in the same house with a beautiful girl of the same age and even meet more beautiful girls through her.
It's like they are to hung up on the idea and forcing themselves to believe they're "Siblings" When the situation could easily be solved, they aren't real siblings after all.
The lesbian girl just seems confused too me. She's kinda selfish since Amane obviously has pure intentions, why would it spring up to love all the sudden? She seemed kinda delusional but looks like there's something going on with the guy friend. They seem like a good couple.