BTW my question on why it is important to believe that Jesus had no brothers was not answered?
I will try to give a short concise answer to this one if I may. Although my answers are seldom short.
There are a few reasons why it is important to believe that Jesus had no brothers. One of the important ones I can think of off hand is to reveal a deeper understanding of the book of Revelation.
In Revelation John is noted as seeing the “Ark of the Covenant” in the sky. This phrase is immediately followed by a detailed description of “The Woman.” Many people who think Jesus had brothers interpret the Ark of the Covenant in Revelation to be the “box” containing the commandments given to Moses. They think John is referring to a box in the sky. We, as Catholics, know that phrase to mean he saw Mary in the sky.
In using the words “Ark of the Covenant” John is showing his
Jewish readers the importance of Mary in her role as Mother of God. The Ark of the Old Covenant was completely
untouched. Those who dared touch it, died. Wherein the tablets were, nothing could be there afterwards, just as nothing had been there before. The Ark’s sole purpose was to carry the Covenant. John is emphasizing that a “mere” box contained the Old Covenant, but a
human carried the New Covenant. That human also remained untouched both before and after she was a vessel of our Lord. The “box” of the Old Covenant was Holy ground. So too was the womb that held the New Covenant. John is showing not only the parallels of the Old and New, but the increase in holiness of the New.
Our late Holy Father in his writings on the Theology of the Body points out that Scripture begins and ends in a marriage. In the beginning is the marriage of Adam and Eve. They were to be the first example of holy marriage. Then they fell. In the end is the marriage of Christ and His Church. Is God implying that Christ and His Bride have sex and make babies? Of course not. He has foreshadowed a more holy marriage of Christ and His Church in the example of Jesus’ earthly family. Revelation is a very difficult book to just pick up, read and understand. There are so many real symbols in it. Who is the new Adam? Who is the new Eve? Who is the Woman clothed in the Sun? If the dragon is waging war “against the rest of her offspring, those who keep God’s commandments and bear witness to Jesus,” is John just saying a few choice men who also came from her womb are being waged against?
The other reason that comes to mind on the importance of believing Jesus did not have other siblings is to not get stuck in “linear thinking.” Many converts I have met really wanted Mary to have had other children later. I discovered in long talks with them that they tended to think in a “before, now, and later,” frame of mind. They struggled with Mary’s perpetual virginity because of the “well he wasn’t in there anymore” kind of thinking. They also struggled with the idea that just because Jesus
used to be tangible 2000 years ago that He wasn’t now. For Catholics and Protestants to dialogue we must remember that Catholic thinking is "everything is RIGHT NOW for God. There is no “yesterday” or “tomorrow” for Him. He is the Alpha and the Omega. Protestant thinking is more linear. They tend to remember a day they “got saved” and a day before that they weren’t saved.
For God, His mother is created by Him. He knew, though, from the very beginning that He
would create her. He gave her free will to say no to being the God bearer, but He knew she would say yes. He is all-knowing. God doesn’t change courses mid-stream. He doesn’t “change His mind” with His creation. Everything is NOW for Him. If Mary
was a virgin to carry Him in perfection then she still is. No one else is worthy to enter her womb.
As I said, not short, but I hope it helps give you a different perspective to ponder.