Responses
I think he did that to tell Shinobu that he should refuse sex if he doesn’t want it (but the nii san got hard so it was a bit weird). Like Nii san was verifying if Shinobu can say no against Kazuomi. The fact that Shinobu didn’t refuse nii san didn’t help at all but Shinobu went to see Kazuomi so i gueee that was enough for Nii San to understand that the sex was consented
In the end, it feels like the brothers didnt love each other romantically, and more like they want to stay with eachother. (Feel free to comment if you think im wrong)