Yes. But neither is conferred upon the birth of a second born. One can be an only child and yet be legally firstborn, and have the benefits and duties of the fact.
One can be an only child and yet be legally firstborn, and have the benefits and duties of the fact.
Other than ignoring the point made, you are now arguing against yourself.
YOU taught us that the context of Matthew 1:24-25 was ONLY about establishing Mary's virginity up to the birth of Christ, and while the theme of establishing that fact is indeed true, you have gratuitously shut the door to deriving anything else from that passage.
So now we come to "firstborn", and while you have already closed the door to any other light to come out, you have reopened the door a crack to say that we now have a theological truth pouring out and that is in the significance of legal and traditional responsibilities, "benefits and duties" assigned to it. The problem with this is that you have elevated the meaning of "firstborn" from that of something slightly more than trivial, to that of a defining attribute. Normally you could get away with that, despite the lack of evidence in the gospel narratives. I'm not denying His role, but the Paraclete through the Gospel writers didn't see fit to make this a factor until we reach the tree where our LORD was slain where he assigned to John the role of caretaker for Mary.
Furthermore, borrowing from some lower criticism, we have the interesting problem in that only the Byzantine texts care to include the "firstborn" language - for it is absent in the Majority texts. If this was a semi-trivial point, this wouldn't be a problem, but if it is elevated as you propose, then we seem to have a drain on our jots and tiddles.
If the "firstborn" language is semi-trivial, then it would support the secondary theme that Joseph didn't touch his wife until after she gave birth to Jesus and that Jesus was the first of several more sons and daughters to come from this legally wed couple. This logical flow of "firstborn" as first of several in v25 is complementary to the "until" of Joseph knowing his wife in v24. Your imposition of a theological attribute in that context is unjustified and is arguably out of place.
Combine these passages with the "adelphi" (sisters)* used in later passages, and the argument pretty much is made that Jesus had siblings.
You have exhausted your Scripture and language based arguments, they have been refuted time and time again. All that is left is for Holy Tradition to trump historical narrative and declare by fiat that which the Scriptures say isn't so.
* the Greek word for "sisters" has NEVER been used to indicate cousins, neioghbors, tribesmen or distant kin and has always been used to indicate out-of-the-same-womb female sibling.