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.
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.
Utsumi smirked because he's planning to emotionally and psychological destroy Akamine by getting closer to him. When Akamine opened up to him, he found a chance to exploit Akamine's mental and emotional state.
I don't think he faked the rape as some claimed, all because of what happened when he did get back at Akamine. "I'm the one who feels like crying." Despite getting back at Akamine, he should feel better by now as he finally gave Akamine what he deserved. However, that's what he believed, but life's not that easy.
He expected for his pain and anger to go away, but it did the opposite - he felt like shit for raping Akamine and he doesn't know why. Hence the "crying" part because he, himself, doesn't know why it didn't go away when it should've. He finally got revenge, and yet the pain is still there.
When Utsumi was saved by Akamine, he said, "Even though you're the same as those bastards! Even though you did the same thing!" Utsumi's view of Akamine shattered as on the one hand, Akamine's this vile, disgusting excuse of a human being. But now, when Akamine helped him out, it shattered. He's lost.
But six years of pain, anger and suffering doesn't disappear overnight, so when Akamine opened up to him, he took the chance.
But that's just a theory