What you are saying is possible. But ... I contend that should Mary not have sex with Joseph she would be doing a bad thing. Paul writes 'do not deny each other, except for a short time of prayer and fasting. Then come together quickly so as to not be tempted'. Mary, by remaining a virgin is subjecting Joseph to a life of temptation. As a man who has been married 12 years I can say from experience that this would be cruel and probably more than he could bear. My second point on this topic is nowhere in my bible have i read Mary remained a virgin. Should this scripture exist I'm willing to look over it. Thirdly the brothers and sisters are never mentioned in the story of Jesus' birth. If they were indeed older siblings one would have expected them to tale part in that journey and some mention would likely have been made. The other three options are: A) siblings were younger but born to Joseph not Mary which would be an odd situation and you would question why Mary is raising them in this situation B) the siblings are kids of someone else but taken on by Mary after a death etc which is possible but would question whether they would be labeled brother and sister or C) the kids of Mary's second husband. But again in this situation we are suggesting Mary is marrying a man then refusing to sleep with him So it is possible but in the balance of probabilities it is more likely Mary slept with her husband as nothing suggests otherwise in my bible.