Catholics believe in single predestination (St. Augustine) not double predestination (Calvin).
Which is to say God know what we are going to do and wills it (permissive will), however He does not force us to do it, we are truly free. God creates beings that can sin because being able to sin means one is able to freely and meritoriously choose. Existence like freedom is an intrinsic good which makes it possible to do good, be happy, and praise God.
God did not create sinful beings, He created beings that are free to love. Freedom in tge Catholic context does not mean the ability to choose evil but the ability, without constraint or interference to choose good. However, the ability to choose Good without constraint necessarily entails the binary alternative, which is fake freedom (aka slavery or sin).
Mary is in fact a special case (preveniant grace). The grace given to her was fitting for the mother of the Incarnate God. But Mary, like Adam and Eve simply had complete freedom in that binary option: yes/no. God knows Mary will not sin because He is outside of time, but inside time Mary is completely free, just as Eve was.
God creates us because He Loves. He loves the Son and the Son loves the Father and their mutual love is the Spirit. Because He loves He creates, creature that are free like He is, but to be free like He is means the ability to choose to be in His likeness or not. Upon giving Eve life He knew she would fall, and yet love kept her in being, because love, being, and goodness are all synonymous–transcendentals which decribe the perfect nature of God. To take Eve out of existence for using her freedom poorly would be unloving, and to know that she would use ot poorly is to give her existence.
So you might ask why God didn’t preserve us all from personal effects of sin. The reason is simple. The effects of sin are the consequence of ill-spent freedom. To negate these effects is to negate freedom itself.
The word prevenient means simply to come before. Certainly there is a grace that comes before baptism. However, Mary’s prevenient grace is akin to the Sanctifying grace of Baptism.
Its not that God can not work in us until baptism, rather He can not work within us until we give Him permission to. In fact, changing people’s wills without permission is contrary to freedom. Grace is constantly offered to man, like Revelation, but like Revelation freedom requires that it have no impact on man unless we consent to it. Just as man can ignore revelation in all its forms–even down to creation–a man must be free to take up grace or ignore it.
God can not work on our will without our permission… this limitation is not a limit on his omnipotence but a self-limitation.
Prevenient grace, as present only in our Lady, gives her the ability to choose, still freely, yes or no. In contrast the rest of us are incapable to choose yes until baptism. The only difference between Baptism and Prevenient grace is timing.
***Timing and (not to oversimplify things), because the timing of prevenient grace precedes conception, Mary never suffered from the effects of sin in the soul–