So your position is that it is not definitive that Jesus had literal brothers and sisters from Mary; He may have, He may not have. The scripture is inconclusive?
How we know that Mary had no other children? We infer it.
And what scripture provides the basis for that inference?
First, theologically, as the totality of human fulfillment inherent in the choice of being the virginal Mother of God would seem to preclude other familial ambition.
That is simply a statement, not a reason. You infer that Mary is virginal because your theological statement says she is virginal.
Second, if Jesus had blood relatives, why would St. John the Evangelist chosen as caretaker?
Why not Joseph's family? Or Mary's sisters, or other relatives? In fact, your own position - they were cousins, not brothers and sisters - would run counter to your own claim that Jesus did not have blood relatives.
Jesus chose John for symbolic reasons, I submit. John was not needed to take care of her physically; John was needed to care for her spiritually. Jesus named them mother and son spiritually, to tend to each others spiritual needs.
Thirdly, it doesn’t seem that the purpose of the marriage of Mary to Joseph was to make babies anyway. When Archangel announces to Mary, she seems perplexed by the suggestion that she would become pregnant by anyone. “I know not man”, she replies. How many young brides do you know who marvel at the idea that they would soon give birth to a great man? She had never intended to be a mother, or to be a consummated wife.
Ummm, Mary and Joseph weren't married when the angel appeared; they were engaged. Mary and Joseph - being good Jews - would not have consummated marriage because they were not yet married. Mary was perplexed because she had not even married, let alone slept with her husband-to-be!
Yes, from Scripture alone we cannot conclude that Mary had no other biological children. We cannot, however, conclude that she definitely had them. It is absurd to suggest so, but not directly contradictory to the scripture.
I agree that the reasons St. Jonh was chosed as Mary’s adoptive son and caretaker was primarily spiritual, in order to confer Mary’s motherhood to all disciples of Christ, and conversely, to elicit her protection of all Christians (see Apoc. 12). It would nevertheless be a violation of the Jewish laws to select a heir who is not a blood relative if a blood relative is available.
Regarding Mary’s reaction in Luke 1: the angel did not suggest the Mary will be pregnant before the marriage to Joseph was to be consummated. Therefore, the logical reaction would have been the same as when any American bride is told that her son will be president of the US one day: “Ah, thanks. I’ll be sure to name the baby Jesus then”. Instead her reaction was “How am I going to get a son in the first place?” It is a very unusual reaction.