No sin or anything tainted with sin can stand in the face of the holiness of God
Does the Holy Spirit indwell you? Have you stopped committing all sin in your life? Are you sinless today, or have you been forgiven of today's, yesterday's and tomorrow's sins through Christ on the cross?
To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory" -- Colossians 1:26-27"Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
Mary was a human being, a sinner in need of a Savior just like the rest of us. She was graced by God to be the human means for Christ's physical birth.
And once Christ was born, she had "Christ within her" in exactly the same manner you and I and all Christians have, by the grace of God alone.
Fini.
Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special wayby anticipation. The essence of original sin consists in the deprivation of sanctifying grace, and its stain is a corrupt nature. Mary was preserved from these defects by Gods grace; from the first instant of her existence she was in the state of sanctifying grace and was free from the corrupt nature original sin brings. Hence, the angel's greeting: "Hail, full of grace, the Lord is with you" (Luke 1:28).
Now we are told that Mary had to be sinless in order to have Christ united to her by a umbilical cord. Why would it be necessary for Mary to be sinless (if the statement made by Hilary and repeated by the Pope is true) when the rest of mankind can be united to Christ while still sinful?