I'm glad to see that most people agree that itsuki deserves much better than airi, most people wouldn't even hear their partner out if they figured out something like this but itsuki had always just been honest and truthful with his feelings towards airi but airi couldn't do the same for him. Im personally someone that thinks that emotionally being invested in someone else and commiting to shit like jerkin off to that person is a form of emotional cheating, so yeah airi is a fcking dick man. I do encourage ppl who find their family to seek out therapy and just figure it out, morally it's definitely not right but that aside you just don't develop romantic feelings for your family out of nowhere, if u do there's something to tackle there like possible grooming, mislabeling emotions. In airi's case I do think it was just the timing of his puberty plus the only person being around his brother who already kinda had a bro complex (non sexual) going around, it seems more like a possessive lust rather than a romantic attraction tbh
Boku no Kareshi no Sukinahito