Well,you can't know your brother's friend more than he does, he might have some hidden things he knows about him and feel he shouldn't be too close to his sister and again, seeing that you admire him and even your friends endorse him to be a prince charming, that he can easily fans or scheme his way to your heart and space and maybe takes advantage of you. I don't think you should misunderstand your brother I believe he just being the big brother that he is trying to protect you from his scheming friend.You know when you admire someone and he approaches you for whatever reason.you might not think straight before failing for it.i think it not in your brother's place to restrict his friend but it better he stops you because you're like his direct responsibility.So see it that way.