As twf said, I do not believe Catholic teaching - or western theology - considers the Theotokos unable to have sinned.
I have a collection of texts from Catholic theologians going back to the 19th century who affirm that Mary, because of her divine motherhood and immaculate conception, possessed moral impeccability, but not metaphysical impeccability, and they hold this to be the common teaching of the Western Church at least since the time of the Scholastics.
Here are a couple of texts from Catholic theologians - the first from the late 20th century and the second from the late 19th and early 20th century - worth reading on the subject:
"Was the Blessed Virgin free from stain because she did not offend God, or because she was impeccable and incapable of sin? The latter is common teaching in Catholic Tradition, while distinguishing it from the impeccability enjoyed by Christ. His may be called absolute and derived from the union of His human nature with the divinity. He could not sin because He was God, and God is infinitely holy. Mary could not sin by reason of an inherent quality, which some place midway between the state of souls in the beatific vision and that of our first parents before the fall.
Concretely this quality may be identified with perseverance in grace as regards grave sin, and confirmation in grace for lesser sins. In either case, however, her incapacity for sin differed radically from that of Christ. Where His was based on the fact that He is a divine person, hers was an added prerogative. It was absolutely necessary that He could not sin, since God is sinless. It was a free gift of God’s mercy that Mary could not sin, but only because she was protected by divine favor."
Fr. John Hardon
The Catholic Catechism
(New York: Doubleday, 1981)
pages 159-160
“Impeccability maybe either metaphysical or moral. Metaphysical impeccability belongs exclusively to God, whereas moral impeccability may also be enjoyed by creatures. It is enjoyed, e.g., by the angels and saints in Heaven. God is impeccable because He is absolutely and infinitely holy; Christ, in consequence of the Hypostatic Union; the angels and saints, by virtue of the beatific vision of the Godhead which they enjoy. How are we to conceive of the impeccability of the Blessed Virgin Mary? It is quite obvious that her impeccability must differ specifically from that proper to God and the God-man Jesus Christ. Here is not a divine attribute, nor is it conditioned by or based upon a personal union of divinity with humanity. It cannot be a result of the beatific vision, because Mary during her sojourn on earth was a wayfarer like ourselves and did not enjoy beatitude. Comparing her impeccability to that of the angels and saints and to that of our first parents in Paradise, we may define it as an intermediate state between the two. It would be asserting too much to say that the Blessed Virgin was capable of committing sin like our first parents; and too little to assert that during her life-time she was incapable of sinning as the angels and saints of Heaven are now, in consequence of the beatific vision. In what, then, did her impeccability consist? We are probably not far from the truth when we assume that God gave her the gift of perfect perseverance as against mortal sin, and that of confirmation in grace as against venial sin. Together with her freedom from concupiscence these two graces may be regarded as the proximate cause of Mary’s impeccability. For its ultimate cause we must go back to the higher and more comprehensive prerogative of her divine motherhood. God owed it to His own dignity and holiness, so to speak, to bestow the grace of perfect perseverance and confirmation in grace upon her from whom His Divine Son was to assume human nature. This idea is aptly illustrated by ‘the woman clothed with the sun’ whom St. John visioned in the twelfth chapter of the Apocalypse. The analogy between Mary’s impeccability and that of her Divine Son would seem to render this theory all the more acceptable, though we must, of course, never forget that the impeccability of Christ is based upon the Hypostatic Union of Godhead and manhood, whereas that of His Mother rests merely upon the grace of divine motherhood.”
Fr. Joseph Pohle
Dogmatic Theology VI
Mariology: A Dogmatic Treatise on the Blessed Virgin Mary, Mother of God
(St. Louis: B. Herder Book Company, 1916)