Counter perspective.
This isn't about Tommy's concern for his sister's welfare and/or whether its a relationship or just sex. This is about Tommy's pride. Its HIS sister, possessive. He treating her like property, territory. He never once asks Josy how she feels or what she wants, that thought never crosses his mind, he doesn't care either. Its all about Tommy and his ego.
Contrast that with Derek, who is more "ewwww... I don't wanna think about what you two are doing" but he also cares about his sister and how she feels. He cares whether she's happy or not and once he realizes that being with the MC and Josy are what Maya wants... he accepts it... just please, no details.
So no, I don't give two shits what Tommy wants or if he got his wittle bitch feelings hurt. If he's so pissed, step up and swing again, we both know what happened last time he tried that (in my game, the MC flattened him and that one sucker punch was the only punch Tommy landed, the rest were all the MC and Tommy hitting the floor). Tommy's a cunt, and mostly useless. Plus in addition to my asshole side that doesn't take that kind of shit off people, I'm also a hopeless romantic, so if I care about a girl or girls... I tend to REALLY care, and I get protective as hell. That Tommy is embarrassing Josy and Maya and making things hard for them and breaking the frat code when that was called... yeah, ass whooping incoming.