Responses
It seemed implied that he did to some extent. Like SA for sure (but not R because he said he didn’t want Yui to be ruined or dirtied or whatever bleh). Also Yui had repressed memories of his brother’s abuse and manipulation that he didn’t remember until the whole water dream metaphor. I think they chose to imply without showing because it would be uncomfortable to see it drawn and also Yui is trying to move on
What was his brother doing to him and touching him for him to say that they aren't in a relationship? Bc I think we forgetting that the psycho brother would literally tie him up and not let him leave, with him a in a vulnerable position I wouldn't put it past the psycho bro to put his hands on him.