Note that Lumen Gentium, N. 56 isn't from the Bible.

The Scriptures never indicate that Mary is divine, rather, "The Holy Spirit will come upon you." (Lk. 1:35) We should note the Bible accurately prophesying that "all generations to come will call" Mary "blessed!" (Lk. 1:48).

She was made pregnant by the Holy Spirit, by God... not by man... she was sinless in how she became the mother of Jesus.
Immaculate Conception - Mary's Sinless Conception and life: Luke 1:28

Luke 1:28 "Full of grace,” in the Greek, "kecharitome" is a perfect passive participle indicating "one (she) who has been perfected in grace,” a past action that is ongoing. See 1 Peter 1, where he speaks of a past action of justification that is ongoing. Our justification in Christ is a past action that is ongoing although it did not start at conception as did Mary's justification by God's special grace, due to her unique role in God's plan of salvation.

This means that her conception was sinless, granted by the Father. Mary wasn't sinless, "For all have sinned and fallen short of the glory of God". She was chosen, not perfect... there's a big difference.

"And it came to pass, as he spake these things, a certain woman of the company lifted up her voice, and said unto him, Blessed is the womb that bare thee, and the paps which thou hast sucked. But he said, Yea rather, blessed are they that hear the word of God, and keep it." [Luke 11:27-28]

Did you see in the above verse how Jesus made of point of not showing any favoritism towards Mary, but rather he pointed people away from her and towards obedience to God.

"And Mary said, My soul doth magnify the Lord, And my spirit hath rejoiced in God my Saviour." [Luke 1:46-47].