Yes brothers and sisters is a term which can mean brothers and sisters in faith. I know St. Paul has used it in that context and so have the priests at my church as well. That’s why you need to look at the context in which the terms are being used.
I do not rule out the possibility that Jesus’s bothers and sisters mentioned in the Bible could really be His cousins-—as some insist. But I doubt it and really it doesn’t matter in he final analysis. I am perfectly content reading the Bible as it is written-—not reading into the Bible or speculating on such matters.
Since Mary and Joseph were in fact a married couple, it is reasonable to assume they did what other married couples do. Physical intimacy between a husband and a wife is a precious gift from God, not a sin or an act of defilement. I see no reason why this gift would have been denied to Mary and Joseph. There is certainly no mention of this in the Bible. While I do concede the possibility of your theory (even though it just that-—a theory) could you EVER concede the possibility that Mary and Joseph consummated their marriage?
It isn’t reasonable to assume, as God was the Father of Jesus, not Joseph. Joseph was to protect both Mary and Jesus, but her body was reserved as a tabernacle for Jesus.
Joseph knew she was chosen by God, set apart. In setting her apart, she would have been physically reserved, so to speak.
Her perpetual virginity is a foil to Eve, as was her “Yes,” to God. Mary played a role in the reversal of man’s condemnation. Hers was to be both an example of obedience, but also provide humanity to Jesus. It was to suffer, as she was foretold by Simeon. It was also to nourish Jesus.
Eve on the other hand, disobeyed God. She helped to bring sin into the world. In the Garden, she suffered nothing. She most certainly didn’t nourish Adam by giving him the fruit of the tree of the knowledge of good and evil. She carried original sin.
Mary carried the light of the world!
Here is a good article that will do a better job than I can explaining. http://www.catholic.com/tracts/mary-ever-virgin