If Mary was perpetually a virgin, why is it that the Bible speaks of the "brethren of the Lord"?
The word "brother" in itself proves nothing.
Besides, there was no word in Hebrew or Aramaic for cousin, so the Old Testament writers were forced to use the word "AH", brother, to describe different degrees of kindred. For example, Jacob, speaking of his cousin Rachel, calls himself her father's brother, rather than style himself the son of her father's sister, the only way he could describe the relationship in Hebrew (Genesis 29:12).
It is certain, then, that if Jesus had cousins, especially if they were born of the same mother, they had to be called His brothers in the Aramaic tongue. Therefore, the phrase "brethren of the Lord" in no way threatens the doctrine of the perpetual virginity.