I think it makes sense. Motoharu purposely acted so recklessly and slept around to make his brother jealous. Then he pretended to be a 'good older brother' like when he apologized for Akio, congratulated him on his relationship, tried to get a job... he was trying to make himself look good so that Akio would realize that his only option to have his brother for himself is to act on his desires and 'rape' him.
In the scene where Akio watches him masturbate you can tell that Motoharu noticed him watching. It also makes sense that he *pretended* to be really drunk, too drunk to remember anything, when Akio and the other guy both fucked Motoharu. Who wouldn't remember such a thing? Don't forget that the other guy said that Motoharu was unusually tight that day and came three times, just because his brother was there.
He knew. He always knew and played Akio like a fiddle - because what does he have experience in? Wrapping men around his finger. You can see it started early when they were both still very young (when the old man offered to give little Motoharu money after their parents passed away).
I think it's clear that the artist planned this all out from the beginning. We only see most of the story from Akio's perspective, so we don't know Motoharu's thoughts at first. But he does act suspicious too, and you can tell that everything he does is to get his brother's attention.
There's probably more that I missed.
Ngl, but that plot twist felt like it came out of nowhere. There was no build-up to it, and worse, it's in the epilogue. It felt very much like an after thought.
I re-read it to see if there were any clues alluding to the twist, but nothing ever really stood out. The only thing I noticed was the flower one in the first chapter. It's all I got.