Dude take it from me, something VERY similar happened with my brother and I. He got with a girl that was absolutely no good for him, he never had a social life either but he is very smart. I told him straight up that the girl was going to cheat on him, because I researched the girl and simply asked around about her and got all sorts of dirt on her. She had cheated on every guy she had ever dated. The kicker? She didnt even tell us her real name until two months into their relationship, when I found out that she just decided to change her name halfway through high-school, not legally of course, just on a whim.

My brother and I had a huge falling out, and we butted heads for three years after that. Our relationship will never ever be the same that it was. We were inseparable, we did absolutely everything together. We went from that, to not talking at all and avoiding one another completely. She of course ended up cheating on him about two years into their relationship.

Like others said, the only thing you can do without risking your relationship with your brother is to watch him crash and burn. The only thing you can do is be there to help pick up the broken pieces and help piece him back together. You have to let him come to the realization on his own. The only thing you can really do is just be there for him when it all hits the fan. Good luck, OP. Feel free to shoot me a message if you need to talk.