She was only treated as all the elect are, their sins are not counted against them. When God looks at me He sees me as sinless, as he has removed my sins as far as the east is from the west and He sees them no more. The old testament saints were rendered sinless looking to the cross in anticipation of the work of Christ.
Mary, like them and us was a sinner that needed the grace of God.
Note that Mary, while free of in, is still "short of the glory of God." So her righteousness is entirely derivative,
As is my righteousness and the righteousness of all the saved
and is not symmetrical or commensurate with His. It is, as has been poetically expressed, like the reflected brightness of the moon in relation to the self-generated brightness of the sun.
Why is it important to you to have a sinless Mary?
Amen, Amen, Amen. That's one nice thing about discussing stuff with people who believe in predestination. Most of the argument just falls into place.
"Why is it important to you to have a sinless Mary?"
Ah, now we're getting somewhere. Because Jesus' sinless human nature was derived entirely from her. As I think you agree, and as the Catechism states, the effects of original sin are the inheritance of our nature.
There's a little more discussion along these lines here.
Just for the sake of clarity, this doesn't make her equivalent to Christ or God. It makes her analogous to Eve --- only when Eve was still in the Garden of Eden, before the catastrophe. Eve was (at that time) sinless, and possessed a human nature as God intended it to be. Call it Human Nature Plan A. She and Adam, however, messed it up, which resulted in a defect in their nature, a defect which was heritable by their descendants. Surely we agree on that?