Mary was his respected loved mother. She is NOT God and we can at most ask her to pray for us like you might our own relatives who passed on.
IMO she is a great advocate to pray for you.
I don’t care about the sin question because in any case Christ has forgiven any sins IMO that would have been that.
Where did Christ or the apostles ever teach that the dead, even if in heaven , can interceded for us?
Where do the scriptures tell you to direct your prayers? (1Ti 2:5)
Scripture tells us that when Christ died, the veil of the temple..the veil that separated men from God, from the Holy of Holies in the Temple ..the curtain that only the High priest could go behind one time a year to mediate for the sins of Israel ...that curtain was ripped in 2, allowing men to have full access to God without a priest. Now we can enter into the throne room of God through Christ..not His mother, not our deceased family members, not the deceased saints.. but through Christ and Christ alone
Hbr 4:16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.