In the New Testament we see the same usage: in Mark 6:3 Jesus is called brother of James, and Joseph, and Jude, and Simon, but in Mark 15:40 it is another Mary who is named as the mother of the fisrt two.
OK, then you do conceed that the Mary listed in Mark 15:40 is the mother of more sons than just Jesus. Because the next argument is that Mary had a blood sister named Mary.
Quite frankly I am tired of hits topic so I am going to play the Ace up the sleeve card.
Matthew 1:24-25 "Then Joseph, being aroused from sleep, did as the angel of the Lord commanded him and took to him his wife, and did not know her till ('έως') she had brought forth her firstborn Son. And he called His name Jesus. "
Joseph "knew" his wife. So much for the virginity.
The "firstborn", 'πρωτότοκον' part is a bit more sketchy since it is found in the Textus Receptus, rather than Tischendorf's or Wescott-Hort. But rest assured, Rome did manage to leave 'primogenitum' in the Vulgate, and thus you need to accept the idea that Joseph and Mary had other sons after her first.
Nor does it say *her ONLY son*. It specifically says *firstborn*.
Mary in 15:40 is the mother of the sons James, and Joseph. She is not Mary the Mother of God -- obviously since the Evangelist descrbing the crucifixion of Jesus would not omit the only relevant relation of that Mary, to Jesus, were that true. Neither is she necessarily a sister of Mary. She is some kid on relative also named Mary.
thus you need to accept the idea that Joseph and Mary had other sons after her first[because of the phrasing of Mt 1:24-25]
Firstborn applies to any first child regardless of whether any other follow. "έως" does not necessarily mean "up till that time but not subsequently". It can simply mean "surely up to this day, and what happens after I have no comment". For example, in the text by the same Evangelist, we read "the field was called Haceldama, that is, The field of blood, even to [eos] this day" (Mt 27:8). Surely St. Matthew did not mean to imply that as soon as the ink is dry the field will be renamed.
The focus in all Nativity narratives is the miraculous birth of Christ. The focus is therefore on the absence of intercourse prior to His birth. The narrator has no comment and does not wish to distract us with what went on later between them.