This depends on which Christian you are asking; the Catholic and Orthodox Churches maintain that Mary remained a virgin even after the birth of Christ, so no, he would have no siblings. But various Protestant churches don't see any issues with Mary and Joseph having children after Jesus. Thus is born the dispute over James the Just and others like him. Apparently, the original Greek can be a little vague on whether someone is a sibling or relative, depending on the word, so it's not quite clear who has the right of it.

Check out this question that was just posted. There are a good number of comments.

http://www.reddit.com/r/AskHistorians/comments/2zhknw/did_jesus_of_nazareth_have_any_siblings/