A friend of mine was kind of in that situation, having a relationship with a muslim girl and having to hide from her older bro (to add detail to the picture, the bro listens to french folk music and serves jailtime on weekends). The thing was going on smoothly for a year or so I believe, and the bro found out recently.
They cant really see each other any more right now, but they're both moving to the same town next year, where the bro will have no control over her... he basically told her she could do the fuck she wanted as long as she didn't see my friend when she came home, thus ashaming her brother if people found out his sister was going out with a non muslim.
This is to say, Robo, that the "brother" problem may not be as clear cut as you think, and his motivations for protecting his sister may be more of a proud/reputation twist than something purely ideological.