The answer is that Mary found favor by God, because she had a pure heart, was untouched by any man, and was God's chosen for giving birth to Yeshua.
The Bible is full of stories where God chose people to fulfill His prophecies, not because they were without sinful flesh but because they had a pure heart that was set on being obedient to God.
Mary was still of sinful flesh, but kept pure prior to the birth and up through the birth of Yeshua. There was no "sin-seed" to pass along from her womb since there was no human father involved at this point. The Bible states that afterwards she and Joseph consummated the marriage and had other children which would have been half/siblings to Yeshua.
The Bible does not state that she and Joseph consumated their marriage, you are adding to the text. I'm sure you've heard many times in your life as a Christian that there are many other relationships in the Bible referred to as brother, thus other possible relationships are being described when in reference to Jesus's "brothers".
Eusebius, who cites the writings of Hegesippus, who lived in the first quarter of the second century, "The same author [Hegesippus] also talks of the beginning of the heresies that arose about this time, in the following words: 'But after James the Just had suffered martyrdom, as our Lord had for the same reason, Simeon, the son of Cleophas, our Lord's uncle, was appointed the second bishop [of Jerusalem] whom all proposed as the cousin of our Lord." (James E. Talmage, The Great Apostasy, Notes, Chapter 6, Note 4, Schisms and Heresies in the Early Church)
Clopas had a wife named Mary and it is their children that are always cited as being the brothers and sisters of Jesus in keeping with the way cousins were often referred to. (Matthew 13:55-56 Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas? And his sisters, are they not all with us? Whence then hath this [man] all these things?)
Mary of Cleophas is mentioned in the King James Version, but more accurately should be Mary "of Clopas", i.e. the wife of Clopas - or Alphaeus. She is brought before us for the first time on the day of the crucifixion, standing by the cross. John 19:25. In the evening of the same day we find her sitting desolate at the tomb with Mary Magdalene, Matthew 27:61, Mark 15:47; and at the dawn of Easter morning she was again there with sweet species, which she had prepared on the Friday night, Mat. 28:1; Mark 16:1; Luke 23:56, and was one of those who had "a vision of angels which said that he was alive." Luke 24:23. She had four sons and at least three daughters. The names of the daughters are unknown to us; those of the sons are James, Joses, Jude and Simon, two of whom became enrolled among the twelve apostles (James), and a third (Simon) may have succeeded his brother in charge of the church of Jerusalem. By many she is thought to have been the sister to the Virgin Mary.
Kiddos calling...more later