With regard to Mary’s virginity after childbirth, it should be noted first of all that there is no reason to think that the will to remain a virgin, manifested by Mary at the moment of the Annunciation (cf. Lk 1:34), has changed later. Furthermore, the immediate meaning of the words: “Woman, behold your son” and “behold your mother” (Jn 19: 26-27), which Jesus directs from the cross to Mary and the Beloved Disciple, suggests a situation that excludes the presence of other children born to Mary.
Those who deny her virginity after childbirth have thought to find a probative argument in the term “firstborn”, which the Gospel attributes to Jesus (cf. Lk 2:7), as if that expression implied that Mary begot other children after Jesus. But the phrase “firstborn” literally means “son not preceded by another” and, by itself, dispenses with the existence of other children. Furthermore, the Evangelist emphasizes this characteristic of the Child, since some prescriptions of Judaic law were linked to the birth of the firstborn, regardless of the fact that the mother had given birth to other children. Consequently, these prescriptions were applied to each only son because he was “the firstborn” (cf. Lk 2:23).
Some who argue against the virginity of Mary after childbirth cite those Gospel texts that recall the existence of four “brothers of Jesus”: James, Joseph, Simon and Judas (cf. Mt 13:55-56; Mk 6:3), and of several sisters.
It should be remembered that, in both the Hebrew and Aramaic languages, there is no particular term to express the word cousin and that, therefore, the terms brother and sister had a very broad meaning, covering various degrees of kinship. In reality, the term “brothers of Jesus” indicates the children of a Mary who was a disciple of Christ (cf. Mt 27, 56), one designated in a significant way as “the other Mary” (Mt 28:1). They are close relatives of Jesus, according to a frequent Old Testament usage (cf. Catechism of the Catholic Church, n. 500).
Thus, Most Holy Mary is the always Virgin. This prerogative of hers is the consequence of divine motherhood, which consecrated her totally to the redemptive mission of Christ.
—from the Pope’s General Audience of August 28, 1996