The parallelism is complete. Jesus was predestined, first  to divine filiation, secondly and consequently to the highest degree of glory and hence to that fullness of grace which belongs to the holy soul of the Word made flesh. Thus too, by the same decree, Mary was predestined first to the divine maternity, secondly and consequently to a very high degree of glory, and hence to that fullness of grace which belongs to the Mother of God, a fullness worthy of the grandeur of her mission, a mission which uniquely associated her with the redemptive work of her Son. .
Mary's predestination, further, again like that of Christ, depends, in the order of material causality, on the permission and prevision of Adam's fall, because, in the actual plan of Providence, if the first man had not sinned, were there no original sin to repair, Mary would not be the Mother of God. But where sin abounded, grace super-abounded.  The Fall was permitted in view of that great good which we see radiating from the redemptive Incarnation,  and Mary, predestined to be Mother of the Redeemer, is thereby predestined likewise to be the Mother of mercy.
Mary's predestination, like that of Christ, is absolutely gratuitous. By no title, either of justice (de condigno) or even of strict appropriateness (de congruo proprie): could she merit divine maternity. This is the common teaching, against Gabriel Biel. The principle underlying this doctrine runs thus: The source of merit cannot itself be merited. Now, in the actual economy of salvation, the Incarnation is the source of all grace, and of all merit, of Mary's graces and of our own.
Further, there is no proportion between merits in the order of created grace and the hypostatic order of uncreated grace. But divine maternity, though it terminates in the hypostatic order, in the person of the Word made flesh, is in itself a created grace. Hence, when we say that the Blessed Virgin merited to bear the Lord of all, we do not mean, says St. Thomas,  that she merited the Incarnation itself. What we do mean is this: By the grace given her she merited that degree of purity and sanctity which was demanded by her dignity as Mother of God. Can we therefore say that she merited the Incarnation, not indeed by justice (de condigno): nor even by strict appropriateness (de congruo stricte dicto): but at least by appropriateness in a wider sense (de congruo late dicto) ? St. Thomas  seems to say so, and is thus understood by many Thomists. The saint's words run thus: The Blessed Virgin did not merit the Incarnation, but, the Incarnation supposed, she merited, not de condigno but de congruo, that the Incarnation should be accomplished through her. This position is in full accord with two other positions: first that she merited our graces de congruo proprio, secondly that Christ merited our graces de condigno.