There is no biblical basis for the immaculate conception of anyone other than Jesus. In fact, in her beautiful song of faith, the Magnificat, she says, “And my spirit has rejoiced in God, my Savior.” Were she without sin, she would not need a savior.
Maybe she didn't know that...
“...no biblical basis...”
The fact that Mary is full of grace is in the bible. Jesus required an immaculate (sinless) vessel for his Divine Nature. Mary required a savior - she was “saved” in advance so her vessel was pure - she is prefigured by the ark of the covenant in the OT.
The state of being full of grace = having been saved.
What the Holy Spirit placed in Mary’s womb was not a DNA gamete result of Mary’s genes and God genes. Jesus was a unique DNA. Since Science now knows that the gestating baby builds their own body, umbilicus, and placenta, Mary willingly provided the sustenance for the gestating Jesus, but He was not carrying Mary genes and God genes, He IS a unique DNA, fashioned by God and placed in Mary’s womb. When implantation occurred in Mary’s womb, she was ‘with child’. Her child was not ‘of her DNA so He inherited none of the sin in Mary’s nature inherited from Adam. Jesus inherited his nature from God alone. How God accomplished this is all that remains for speculation. There are several possible ways God could do it, but none include DNA from the descendants of fallen Adam.
Yes. Mary was used to fulfill a miracle. Lazareth participated in a miracle as well and no one claims he was sinless. All of this detracts from the focus which should be on Jesus Christ.