No one said anything about who gave birth to Jesus. Catholics like to point to the verse that says Mary was blessed among women and hold that as proof that she was sinless. Clearly there are other passages in scripture that would refute that interpretation of that verse.
I prefer to point to "filled with grace"
Luk 1:28 και εισελθων TSBο TSBαγγελος προς αυτην ειπεν χαιρε κεχαριτωμενη ο κυριος μετα σου TSBευλογημενη TSBσυ TSBεν TSBγυναιξιν
In the Latin
Luk 1:28 et ingressus angelus ad eam dixit have gratia plena Dominus tecum benedicta tu in mulieribus
When you are "filled with grace", there is no room for sin.