Short Answer: No, Mary was not sinless.

Explanation: Mary was NOT sinless. No human has ever been sinless or ever will be sinless, Jesus being the only exception due to him being wholly God and wholly man.

Similarly how God can use a person to lead another person to Him, God chose to use Mary to give birth to Jesus as a virgin because that's how His plan unfolded.

Romans 3:23 - "For all have sinned and fall short of the glory of God."

Romans 3:10 - "as it is written: There is no one righteous, not even one."

Ecclesiastes 7:20 - "There is certainly no righteous man on the the earth who does good and never sins."

The Virgin Mary, mother of Jesus, was just an ordinary person like you or me. I think she was about 13 or 14 years old around the time this happened. But then God intervened in her life and gave her a special task to bear: to give birth to Jesus. Sure, she birthed the Son of Man's human body. But that didn't make her sinless.