Maybe that's because you're thinking as how someone in real life would react but in fiction not everything is as in real life, is like asking yourself why on Harry Potter they have Magic when in real life we don't (maybe it's not a good example but anyways)
I'm just trying to say that yes, i would be gross out in that situation in real life but in fiction they are just 2D characters, i'm not saying you should like this i haven't even read it, just saying fiction not always make sense or not always need to be as in real life.
Yea i know i don't care much abt morals in fiction but the point here is that this relationship between two brothers is handled pretty realistically it doesn't feel that fiction-like to me so i can not help but project myself onto the situation lmao this is why i dislike fiction in modern settings i end up trying to make sense of things
This makes sense except the incest plus why is the big brother okay with the romantic feelings his younger brother holds for him like normally you should be grossed out or smth