I SERIOUSLY WANNA BEAT HER UP

I’m usually a very civilized person but I guess not when it comes to my baby brother. He’s 18 and just got out of a two year relationship with a girl. They broke up in August but he didn’t tell us til Thanksgiving. And even when he told his he wouldn’t tell us why they broke up. They still are very close, my parents said that she comes over now more than she did when they were dating. Anyways, me being super nosey like I am, I’ve been trying to find out why they broke up. Only a few people at my work know why, and I FINALLY got one of them to spill the beans. Apparently she had sex with another guy and that’s why they broke up. Alright so wooo go my brother for leaving her ass.

Well after I find out I’m talking to my dad (he’s in prison and we don’t talk to him often) about it on the phone and he’s like I thought that’s why they broke up the first time. Of course I knew nothing about them ever breaking up before. But apparently she cheated on my brother before, sending naughty pics and having sex with other guys. Like wtf. And my mom tried to talk to my brother about not staying with her or something I guess. And he told my mom “she’s my best friend, I really care about her.” And of course that breaks up heart because honestly he’s probably not over her yet. He still has pictures of her all over his bedroom wall, and sees her all the time. I just wish I could talk to him about it, but he doesn’t know that I know. Sooo I guess the only logical thing is to tell this girl to keep her hands off my brother or keep her legs closed.

Any ideas of how to confront him? Idk I just feel like I need to keep him from getting hurt again.