Before Jesus could take flesh for our salvation, He needed a mother. But, not just any mother. So, Jesus conceived of Mary in the Divine intellect before He created her, through her parents, in the flesh (John 1:1-3). Yet, He could not assume a sinful flesh, since God cannot coexist with sin - especially in a hypostatic union. So, Jesus created His mother, by a singular grace, and in light of her specific mission, to be also without sin. Mary was saved before she knew it - at her conception. We know that sin passes through the blood, and we know that Jesus is sinless.
Mary, in turn, conceived of Jesus in her womb to give her flesh to Him. Jesus had to have a blood connection to the line of David in order to fulfill prophecy as the Messiah. So, He obtained that connection through the flesh of the mother that He created for Himself. Look at sin and redemption this way: The devil tempted two sinless human natures to fall from grace, so God used two sinless human natures to restore man to grace.