In God's economy, the nature of descendancy is based on the father, not the mother.
You are king if your father was of the kingly line, not your mother.
You are without a sin nature if your father, not your mother, was without a sin nature.
Jesus' father was God, therefore, Jesus had no sin nature.
Before we go much further please tell me what "sin nature" is and how we were made evil. Why did God find Adam and Eve culpable of being evil after He made them evil. Just to be clear we are talking about the same thing, nature is the innate common attributes all men have. Nature is "The essence of a being considered as the principle of activity."
That is a doctrine of man.
Nature is how God created us. Men had nothing to do with it.
There is nothing in the NT that states Mary was ever sinless.
There is nothing in Sacred Scripture that says she wasn't, and everything to say she was.
Mary's legacy to us magnifies the Lord, literally. It is through her we see the “Real Presence” of our Lord. We see that Mary overshadowed by the God, her spiritual spouse. Christ’s conception is miraculously without original sin, thus the “seed of man". It was Mary’s immaculate soul and most perfect flesh (without original sin - thus a New Eve) which Christ bore. We can't say that it was God's flesh that he bore else he would not be man and we know Christ to be wholly God and wholly man uniquely and inseparable joined. Further, as mother of the Lord, she becomes by right of motherhood, queen mother of her Lord, [Apocalypse of St. John 11:19]. This role can only rightly be taken by Mary who intercedes for the citizens of the Kingdom of God. Mary is the only figure in Scripture addressed by an emissary of God, "Hail Mary" receiving foreknowledge that her yet to be conceived Son is God. God's emissary's words are spoken as if addressing a queen, “Hail”. This is the language seen in the courts of kings and queens. Then Gabriel goes on to say of Mary, “kecharitomene”. This Greek word is translated as “most favored or highly favored” and from the traditional Latin translation to English “full of grace“ or in Latin “gratia plena”. What it means however is that the woman being hailed is “
completely, perfectly, enduringly endowed with grace." [Blass and DeBrunner, A Greek Grammar of the New Testament and Other Early Christian Literature].
She was called "favored one" by the angel, but that does not make her without Adam's nature.
That's one of two interpretations I'm aware of. What difference does it make? She remains “
completely, perfectly, enduringly endowed with grace."
Abraham was favored as the origin of God's people, but that does not mean he was sinless.
And He is adored as Father Abraham by Jews and Christians alike.
Then you are in disagreement with the NT, particularly Ro 7 (v.23).
That baby rattlesnake in my backyard where my children play didn't ask to be a rattlesnake, but he is culpable nonetheless, and I will kill him.
St. Paul is indeed discussing our nature in this verse but the nature of sin. Sin is a voluntary immoral word, act or desire. These are acts contrary to the laws of reason. We aren't unthinking animals, God made us rational beings. The result is not only corruption to death, sin therefore elicits disordered desire, a loss of original justice.
St. Thomas Aquinas discusses, posited law, Eternal law Divine law, natural law, human law, Divine law, Divine law, and law of sin. I'd be happy to discuss them
JoeT