Think about this people. If this brother's friend's name is not on the lease to the house/apartment/whatever, then he is not legally permitted to live there. As long as you don't want him to live there, he can't, because your name is already on the lease and his isn't. It's as simple as that, just tell you brother that your going to get him evicted for breaching the lease that you both signed when you moved into your apartment. He's not legally allowed to just bring people into the house to live....
Peace