What's wrong with liking your best friend's brother? I think it's OK to like anyone you want to. Be respectful, honest, caring, and have a willingness to talk about anytything, and you will have friends.
I'm kinda in the same thing you are right now . My best friends brothers really sweet and cute but I'm afraid to talk to him cause I'm afraid to lose my best friend if we brake up so I guess just try to stay away from him :)