He is without sin because He is God. The better question is ... how could she be without sin?
SnakeDoc
But he’s not just God. He’s God and Man.
Why didn’t he inherit a fallen human nature like the rest of us, which he would work to overcome?
“how could she be without sin?”
The same way that God made Adam and Eve without sin. Christ kept Mary from sin at her immaculate conception so that she, as his mother, could pass on a perfect human nature, like Adam and Eve had so many years ago.