I guarantee you that if after the sacrifice of the firstborn calf, that was the only calf the cow ever had, the farmer would certainly note that to anyone who cared. The New Testament Gospels were written after the writers would have known whether Jesus was the one and only child born of Mary. I may be shallow in my position, but it doesn’t make me wrong. That Mary & Joseph would not have lived as husband and wife after the birth of Jesus is pure nonsense with absolutely nothing to support that contention. The only evidence to support no siblings is what Jesus said to Mary at the Cross - an unnamed disciple (John, reputedly) was charged with taking care of Mary - if there were other children, she would have lived with them - however, I believe that there was a deep schism in the family, probably after Joseph died, because they didn’t believe in Jesus until after the Resurrection.
You are entitled to your opinions, to go as deep and scholarly as you want to, but the Bible does not say what you or Cronos are saying. Cronos quotes Luther, who was a Roman Catholic who left the church due to other “add ons.” If he and others want to believe that Mary remained a virgin throughout her life, knock yourself out — all that does is contribute to her deification, which is wrong IMHO. There was no requirement for continued virginity from God, or I think He would have mentioned that.
So you agree that "first born" does not mean there must be a second born. Thanks for obliterating your own feeble argument.