No the two brothers aren't fucking. The father is hinted as being the rapist. The middle brother looks more like the dad so I think that's why the older brother doesn't like when the middle brother touches him. But the middle brother doesn't know what the dad did that's why he's thinks the younger adopted brother is psycho for attacking the older brother. The older brother knows what happened to the younger brother because they went through the same thing. It's a shared secret but the older brother is fuckinf weird about it. So something he said or did may have been the reason why the younger brother attacked him. The middle brother doesn't know anything and thats probably why the older brother keeps telling him to be kinder to him.
if you don’t mind being spoiled….. [SPOILER] when the MC gets brought back to the house, the two brothers talk outside of his door and the older brother like caresses the younger brother’s face (who blushes) and the MC sees this and goes “aren’t the two of them supposed to be brothers?” which. i think has a lot of implications.
so we all know the two brothers (of our MC) are fucking right? like they are definitely not normal (also i hate them! sorry im just a simple reader they’re assholes) i’m pretty sure the older one is adopted as well (just a guess tho) but im not 100% sure i believe he was also sexually assaulted like the MC was, i wouldn’t put it past this manipulative piece of shit to be lying about it