The following quote explicitly states that Mary was excepted from committing sins on account of her privilege of being the Theotokos. Obviously, the doctrine of Original Sin comes to full fruition in the writings of St Augustine. And yet even at this source we find a very strong reluctance to explain Mary's moral status as anything but sinless.
"Having excepted the Holy Virgin Mary, concerning whom, on account of the honor of the Lord, I wish to have absolutely no question when treating of sins - for how do we know what abundance of grace for the total overcoming of sin was conferred upon her, who merited to conceive and bear him in whom there was no sin?--so, I say, with the exception of the Virgin, if we could have gathered together all those holy men and women, when they were living here, and had asked them whether they were without sin, what do we suppose would have been their answer?"There is one thing to be noted here. St Augustine, in this passage, refers to Our Lady as without "sins" in the plural. He is not talking about the inherited Original Sin, or so it seems. All that can be concluded is that she was without "sins," by which I understand him to be referring to actual sins and Original Sin. Am I right on this?
(Nature and Grace 36:45)
Compare this quote to St Ephrem and St Ambrose who explicit refer to Our Lady as being without "stain of sin."
"Thou alone and thy Mother are in all things fair, there is no flaw in thee and no stain in thy Mother."It seems that St Ambrose was much more explicit in his definition of Mary as without the stain of sin.
(St Ephraem, Nisibene Hymns, 27:8)
"Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free of every stain of sin."
(St Ambrose, Sermon 22:30)