Mary was saved by faith and her salvation came through her earth born son Jesus Christ whom she carried. Mary was born a sinner {under the curse} just like the rest of us were since Adam and Eve's children.
Mary was a chosen one of GOD like Noah, Abram, Jacob {Israel}, Moses, David, Solomon, John The Baptist, Matthew, Luke, Simon, Paul, Pastor Smith and Father Jones, was called upon and did the Will of GOD asked of them. Mary was blessed because of her obedience and faith {which GOD obviously knew her heart} GOD revealed to her the Savior the same way he revealed him to Simon Peter.
Simon Peter too was blessed does that mean he too was sinless? No. It just means he was blessed by a revelation the Lord gave to him revelation of Jesus Christ as Lord and Savior being the New Covenant. He was blessed because GOD revealed His Son his plan of salvation to him not by the acts of men but by His Spirit telling him This is The Christ The Messiah promised. Peter was blessed in the same way we all now are.
GOD finding favor in someone does not mean they are sinless. Only Christ was sinless and only because in Spirit being {his very being} He is the Son Of God and all things were given to Him to overcome the curse upon man in His earthly task.
Believe what you want