And the angel being come in, said unto her: Hail, full of grace [κεχαριτωμένη], the Lord is with thee: blessed art thou among women.
To be full of grace to such an extent as to conceive the 2nd Person of the Holy Trinity is incompatible with sinning.
From the Haydock Commentary on Luke 1:28:
Hail, full of grace:  by the greatest share of divine graces granted to any creature. This translation, approved by the ancient Fathers, agrees with the ancient Syriac and Arabic versions. There was no need therefore to change it into gracious, with Erasmus; into freely beloved, with Beza; into highly favoured, with the Prot. translators. For if seven deacons (Acts vi. 3.) are said to be full of the Holy Ghost, as it is again said of S. Stephen, (Acts vii. 55.) and also of the same S. Stephen, (Acts vi. v. 8.) that he was full of grace, (as the learned Dr. Wells translates it in his amendments made to the Prot. translation) why should any one be offended at this salutation given to the blessed mother of God; who would not have been raised to this highest dignity, had not her s\oul been first prepared for it by the greatest share of divine graces? — The Lord is with thee, by his interior graces; and now, at this moment, is about to
confer upon thee the highest of all dignities, by making thee truly the mother of God. Wi. — The Catholic Church makes frequent use of these words which were brought by the archangel from heaven, as well to honour Jesus Christ and his virgin Mother, as because they were the first glad tidings of Christ's incarnation, and man's salvation; and are the very abridgment and sum of the whole gospel. In the Greek Church, they are used daily in the Mass. See the Liturgy of S. James, and that of S. Chrysos.
Pope Pius IX gives in his dogmatic definition on the Immaculate Conception, Ineffabilis Deus (1854), many reasons for Mary's sinlessness. Here's what he says about Luke 1:28 and Luke 1:42:
When the Fathers and writers of the Church meditated on the fact that the most Blessed Virgin was, in the name and by order of God himself, proclaimed full of grace [Cf. Lk 1:28.] by the Angel Gabriel when he announced her most sublime dignity of Mother of God, they thought that this singular and solemn salutation, never heard before, showed that the Mother of God is the seat of all divine graces and is adorned with all gifts of the Holy Spirit. To them Mary is an almost infinite treasury, an inexhaustible abyss of these gifts, to such an extent that she was never subject to the curse and was, together with her Son, the only partaker of perpetual benediction. Hence she was worthy to hear Elizabeth, inspired by the Holy Spirit, exclaim: "Blessed are you among women, and blessed is the fruit of your womb." [Ibid., 42. ]
He also says:
As if these splendid eulogies and tributes were not sufficient, the Fathers proclaimed with particular and definite statements that when one treats of sin, the holy Virgin Mary is not even to be mentioned; for to her more grace was given than was necessary to conquer sin completely. [Cf. St. Augustine: De Natura et Gratia, c. 36. ] They also declared that the most glorious Virgin was Reparatrix of the first parents, the giver of life to posterity; that she was chosen before the ages, prepared for himself by the Most High, foretold by God when he said to the serpent, "I will put enmities between you and the woman." [Gn 3:15]—unmistakable evidence that she was crushed the poisonous head of the serpent. And hence they affirmed that the Blessed Virgin was, through grace, entirely free from every stain of sin, and from all corruption of body, soul and mind; that she was always united with God and joined to him by an eternal covenant; that she was never in darkness but always in light; and that, therefore, she was entirely a fit habitation for Christ, not because of the state of her body, but because of her original grace.