Mary did not need to be sinless so that Jesus could be sinless. Jesus is God and therefore sinless.
Mary, as the Ark of the New Covenant needed to be pure in order to hold the Word of God.
The Old Testament speaks of people being killed if they inappropriately touched the Ark of the Convenant, which contained the written word of God given to Moses and some of the manna in the desert. The Ark's construction was directed by God Himself to precise specifications and of the most precious materials available to the Israelites.
Fast forward to the Incarnation of Our Lord, who is the Word Himself and the "Bread of Life" come down from Heaven. Are you saying that God would treat His Mother (and Mary is His Mother because Jesus is God) in a different fashion than He treated the Ark of the Covenant?
She was pure. Sinless and pure are two different things. The gold that was used in the ark of the covenant was not FOUND perfect. It was refined. Mary had relationship with God and loved Him. She found favor in with God because of her faith not because she was perfect.
I have heard others say that Mary had to be sinless because if she had sinned she would have passed on this sin to Jesus.
Coul it be that the instant that God placed Jesus in Her womb, she was instantly sinless. I mean if one understands the gestational process, the child is making their own body of organs and tissues, but shedding cells into the amniotic fluid and these taken away via the umbilicus via adsorption. Mary would have been ‘under the blood of the Savior’ while Jesus was gestating in her womb.