I thought I heard that (some?) Catholics believe Mary was sinless, as Jesus was, too. Is that right?
See Post 68 for an explanation.
Yes mary is sinless. But this is not to compare her to Christ. Jesus is sinless because He is God. Mary is sinless only because Jesus by His grace made her sinless at the moment of her conception. She is not sinless for any reason but that of God's grace. She remains a creature who had to be saved from sin by the Grace of Jesus Christ. The only great difference is that this saving took place when she was conceived.