She was a humble servant of God, and favored above all other women because God chose her to bring bring about The fleshly manifestation of God — Jesus!
As such, this does NOT require Mary remain a virgin AFTER the birth of Jesus. In other words the divinity of Jesus is NOT attacked by the humanity of Mary. Indeed the divinity of Jesus is derived from the father (God, though the Holy Spirit).
It seems sin is inherited or visited upon the children through the father, not the maternal side. (there are scriptures to this point). Thus since the father is God, Jesus can come into the world sinless. Mary not need be sinless her entire life to be the vessel that bore Jesus. However Mary was indeed a virgin we know at least until Jesus was born. Since the bible indicates Jesus had brothers, we can deduce that Joseph was the father of Jesus’s siblings.
This in no way impugns the nature of Jesus, or Mary.
Which raises another question which has been asked before and never been answered by a Catholic.
Just why is it so important that Mary be perpetually a virgin?
Why is it so important that she never had a normal marriage to her betrothed and never had sex or other children?
Just why is it so important that Mary be perpetually a virgin?
Because we believe it's true, THAT'S what makes it important.