What's up with my friend's brother?

Sounds like he likes you. ALOT. You should sit him down and ask him to be honest and tell you if he likes you. If he says yes then tell him you dont like him like that and just want him to stop being mean to you and want to be friends. If he doesn't tell you then he either is too shy to admit it or he hates you. Most likely he likes you. If he doesn't stop then go to his parents about the situation. He is teasing you in a way he sees as ok but it's not to you and it's hurtful. If you tell him that it hurts you when he does that then most likely he will tone it down a bit. Hope I was able to help and hope he let's up on ya. (: