Devils advocate here. So today I was scrolling the meth subreddit. Just completely enthralled. If my partner looked over my shoulder and thought I was a methhead over it, that would be an issue on how well he knows me.

If a girl in this situation is scrolling a bisexual little sisters instagram a man wouldn't bat an eye. It would just be checking up on someone you mutually care about.

Him blowing up on you could be a release of things you are obviously insecure about and he can see it. He can see you are worried his touchy feely actions might mean he likes him, he might actually mean it as a big brother thing. He knows you caught him looking at his instagram and acted weird about it, so maybe he could have drank more today knowing you were going to be weird about everything. Maybe he's mad at you for acting weird about him and your way youngwe little brother, not the fact little brother has a boyfriend. You might be insecure about friends giving your brother attention in the past and are insecure because little bro is good looking. Idk. Only you know. Reddit doesn't know that's for sure.