The only requirement for Mary in Scripture was that she be a virgin until after she gave birth. The prophecy in Isaiah was that a virgin will conceive and bear a son.
It happened. Prophecy fulfilled.
The fact that Mary DIED is proof enough that she was subject to the FULL penalty of sin. Rom. 6:23, Gen. 2:17, 3:19.
Which leads to the next point. If the Virgin Mary had an inherited sin nature (which she most certainly did), she could not bring forth a pure offspring.
"The Holy Ghost shall come upon thee, and the power of THE HIGHEST, shall overshadow thee; therefore also that 'Holy Thing' which shall be born of thee shall be called the Son of God." Luke 1:35.
I think this is pretty clear that the embryo that was deposited in the womb of the Virgin Mary by the Holy Ghost contained no taint of sin, and that Mary's womb was SIMPLY THE VEHICLE FOR THE FORMATION OF THE HUMAN BODY OF CHRIST. Into which the "Spirit of Christ" entered AT BIRTH and THUS WAS FORMED THE GOD-MAN.