Yee, like, we get the backstory but these guys dont acknowledge anything other than saying "aniki," which isn't much, bc, yakuza. But, like, even dad's just chilling. No internalized shame or morals just bros dicking bros.
Maybe one of them knows a secret (that they're not related)? So, its green lights in his head? Or this is just a trope checklist for the author? But I agree that it's well written and definitely thought out, so that also threw me off too??
Without spoiling too much, I can say that I think the incest trope was added because, due to his earlier trauma, Yutaka can now can only have "plays" with his brother because Kazunari is the only one he trusts. In the second volume, which I'm currently translating, you'll learn much more about the background and also about the inner thoughts of both characters.
I don't really get how to feel bc there is like zero acknowledgment of the incest. maybe this is a world where dom/sub instincts have made it so sex is trivialized in a way and intimacy between siblings is not as transgressive but there has been nothing to indicate that and we don't really see inside the older brother's head to find out why he is so nonchalant about initiating sex via play with his younger brother. It's confusing to me because otherwise it seems well written and I like the pacing and the characters but the incest element feels random and poorly thought out