“The only sinless person born was Mary, the mother of Jesus”
Huh? Then why could not she have borne our sins at the cross? What need was there for the Holy Child if we already had one without sin? Only 2 have been born free of sin: Adam and Christ; hence Scriptures’ references to Christ as the Second Adam. The first Adam, through disobedience, brought the stain of sin upon all humanity and Christ (the Second Adam) through obedience, broke the power of sin over mankind.
Jesus had some of Mary’s DNA which obviously makes Mary without sin as was Jesus, both God and man.