I feel that but here is what I got. Nissan maybe liked shinobu (he was even hard touching him) but he realized too that his little brother can't oppose him. Shinobu just was letting him do whatever he wanted, it wasn't like he liked the situation, he didn't clearly understand why Nissan was touching him. And as you said, that was a test too, because he the realized that things were different with kazuomi, because shinobu isn't that passive with him, in the last chapter with even seem him asking for sex.
I'm glad this didn't turn into a competition between father and son, I'm glad niisan realized it was time to let his brother go. And shinobu is finally growing thanks to that, in the last chapter he even looks like and adult.
I feel there were many mistakes in this story but at the same time this is the first story by this mangaka, it was poorly executed but it wasn't a bad idea perse.
My phone autocorrected a lot of words, sorry.
Ugh yeah his dick was hard, I interpreted that as something which is a natural occurrence because they were doing something sexual. As much as possible, I don't want to think that his brother also loves him because that's just ughhh but if I think about it objectively, it's more likely that his brother also has feelings for him. Well, no matter what happens, Shinobu will end up with Kazuomi anyway let's just leave it as that HAHAHA
I think the brother noticed by now that Shinobu also likes Kazuomi? I don't think the brother likes Shinobu romantically, he was just testing his feelings in a way? Seeing that Shinobu still can't oppose him, he realized that Shinobu still hasn't fallen for Kazuomi that hard. Fuck to be honest I feel like I'm just forcing everything to fall into place. Anyway that's what I think because the manga ended with a note that Kazuomi will do everything so that Shinobu won't be able to think twice about leaving with him next time.