"[Some] insist that "brethren of the Lord" must be interpreted in the strict sense. They most commonly make two arguments based on Matthew 1:25: "[A]nd he did not know her until (Greek: heos, also translated into English as "till") she brought forth her firstborn son."
They first argue that the natural inference from "till" is that Joseph and Mary afterward lived together as husband and wife, in the usual sense, and had several children. Otherwise, why would Jesus be called "first-born"? Doesn’t that mean there must have been at least a "second-born," perhaps a "third-born," and so on?
But they are using a narrow, modern meaning of "until," instead of the meaning it had when the Bible was written. In the Bible, it means only that some action did not happen up to a certain point; it does not imply that the action did happen later, which is the modern sense of the term. In fact, if the modern sense is forced on the Bible, some ridiculous meanings result.
Consider this line: "Michal the daughter of Saul had no children till the day of her death" (2 Sam. 6:23). Are we to assume she had children after her death?
There is also the burial of Moses. The book of Deuteronomy says that no one knew the location of his grave "until this day" (Deut. 34:6). But we know that no one has known since that day either.
The examples could be multiplied, but you get the idea—nothing can be proved from the use of the word "till" in Matthew 1:25. Recent translations give a better sense of the verse: "He had no relations with her at any time before she bore a son" (New American Bible); "He had not known her when she bore a son" (Knox).
The Lord put off these hereditary evils from her within himself all his natural life.
Mary's place in Heaven is much like any other.