The question whether Mary remained a virgin her whole life appears to be answered here:
Matthew 1:24-25 (KJV) Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
The marital relationship between Joseph and Mary here indicates that they did consummate (have sexual intercourse) their marriage after the birth of Christ. That would mean Jesus would undoubtedly have had half siblings through Mary
Whenever the Koine Greek word "
ἕως"
(heōs), translated to
"until" in English, is used in Scripture, it's not always used to indicate what didn't happen until a certain point started to happen after that point. In some cases, it's used to refer to something that didn't happen during a certain period, like in the case of Matt. 1:25.
"But while he thought on these things, behold, the angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is
conceived in her is of the Holy Ghost. And
she shall bring forth a son, and thou shalt call his name Jesus. For
he shall save his people from their sins. Now all this was done that it might be fulfilled that which the Lord spoke by the prophet, saying: Behold,
a virgin shall be with child and
bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. And Joseph, rising up from sleep, did as the angel of the Lord had commanded him, and took to him his wife.
And he knew her not until she brought forth her firstborn son, and he called his name Jesus."
In Matt. 1:20-24, the author is speaking about the long-anticipated messianic prophecy coming to fruition, and Joseph accepting as his spouse
the virgin who
conceived the Savior of mankind
by the Holy Spirit. In Matt. 1:25, the author reiterates and reinforces that the Savior was truly begotten by the Holy Spirit, and born of the Virgin Mary, by stating that Joseph, whom he had just said accepted Mary as his wife, didn't have sexual intercourse with Her prior to Jesus's birth, to dispel any belief he was conceived by him, and not born of a virgin. The author's entire focal point is on the messianic prophecy, not that Joseph finally got to have sex after the Savior was born unto the world...
Your argument that the author concluded speaking about the long-anticipated messianic prophecy coming to fruition, and Joseph accepting as his wife Mary,
the virgin who
conceived the Savior of mankind
by the Holy Spirit, by ending with this tidbit: "After the birth of the Savior, Joseph finally got to have sex", is quite random, and doesn't fit with the context of Matt. 1:20-24.
Additionally, the word
"πρωτότοκος" (prōtotokos), translated to
"firstborn" in English, has more than one definition.