Mary, the Mother of God, is not part of the Trinity. The Church teaches that at the instance of her conception, Mary was provided a singular priviledge and grace of the omnipotent God, in consideration of the merits of Jesus Chirst, the Savior of mankind, and so was preserved from all stain of original sin. This statement by the Church affirms (1) this immunity was a special grace from God, (2) through the foreseen merits of Christ, (3) Mary was exempt from original sin contracted by the rest of mankind, and (4) the exemption took place at the first moment of her conception in the womb of her mother.
This one boils down to how much credence you put in the traditions taught by the Catholic Church, vs what is found in Scripture.
The plain language in scripture indicates Mary was a sinner saved by her Son, and that she had other children. If one gives at least equal weight to Catholic traditions, then you CAN explain away the plain meaning with what is a bit twisted, but not impossible.