Hey, I would suggest not taking it any further than friendship with your brothers girlfriends brother as it can get weird and difficult. But being his friend is fine.
You must consider why you and your ex broke up, if there's problems there then how do you know they wont appear again if you go out again. But he's your ex for a reason you might wanna just try and forget about him which i know sounds bad but it can be the best thing to do.
You shouldn't really be stressing about boys anyway, your young and have plenty of time to be in a relationship later on.
D.Angel