The best I can do is give you a cradle Catholic answer. For others unfamiliar with this Teaching, it states:
The most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin.
In the same way the Ark of the Covenant was immaculate so
it could hold the
Ten Commandments, so Mary, who gave birth to God-incarnate — Jesus, was immaculate.
Mary was not saved on her own merits. She was saved preemptively or in anticipation of the merits of Jesus' Death on the Cross.
If you and your wife went to Church on a rainy Sunday and your wife's mother
said that because it's raining you should probably take the car, you could either:
- Take the car and get there dry, or
- You could walk and, thinking you could get there dry, accidentally have some muddy water
splash up on your Sunday bests. That said, you could still wash the mud
and dirt off from your pants and her dress respectively, and arrive
at the church clean as a whistle for services.
In both cases, you arrive clean as a whistle. In the first case,
your mother, in anticipation of the bad weather, saved you from getting
mud and dirt on your clothes.
In the second case, the mud and dirt (original sin) was washed away
(through Baptism) by your efforts before you reached the Church.
What I've tried to give you is an earthly analogy of the Immaculate
Conception which took place in the womb of Mary's mother: St. Anne.
Many mistakenly think the Immaculate Conception took place at Christmas, when Mary gave birth to Jesus; it did not. Catholics refer to the Christmas event as Mary's Virgin Birth because she remained a Virgin, before, during, and after the birth of Jesus. It was no ordinary birth — it was the Miraculous birth.
This teaching does not change the fact that the Blessed Virgin Mary was completely human.
This includes her free will to choose good or evil.