You should really read the scriptures that prophesize about the coming of Christ in the old testament.
It makes it clear that Mary is not simply some woman. (Though it doesn't say anything about her being 'without the stain of the original sin', mostly because there is no old testament notion that the guilt of the original sin is passed along)
The issue appears to be whether Mary was immaculately conceived, even though there is no hint in scripture that that is the case. (You might have to ask how Mary could be immaculately conceived if her parents weren't immaculately conceive, and their parents, and their parents....and so on.) At some point, one needs to jump into the lineage with a sinless person. The scripture warrants it with Jesus....the prophesied Messiah of God....and only with Jesus.
As to Mary's lineage, we have scripture saying that the Messiah had to come from Jacob....David...etc. Both were definitely poor sinners saved by grace.