[...] As regards her virginity after giving birth, it must first of all be pointed out that there are no reasons for thinking that the will to remain a virgin, which Mary expressed at the moment of the Annunciation (cf Lk 1:34) was then changed. Moreover, the immediate meaning of the words: "Woman, behold, your son!", "Behold, your mother" (Jn 19:26), which Jesus addressed to Mary and to his favorite disciple from the Cross, imply that Mary had no other children.
Why is Jesus called Mary's firstborn?
Those who deny her virginity after the birth thought they had found a convincing argument in the term "firstborn", attributed to Jesus in the Gospel (Lk 2:7), almost as though this word implied that Mary had borne other children after Jesus. But the word "firstborn" literally means "a child not preceded by another" and, in itself, makes no reference to the existence of other children. Moreover, the Evangelist stresses this characteristic of the Child, since certain obligations proper to Jewish law were linked to the birth of the firstborn son, independently of whether the mother might have given birth to other children. Thus every only son was subject to these prescriptions because he was "begotten first" (cf Lk 2:23).
Several degrees of relationship are implied by the term 'brother'
According to some, Mary's virginity after the birth is denied by the Gospel texts which record the existence of four "brothers of Jesus": James, Joseph, Simon and Judas (Mt 13:55-56; Mk 6:3), and of several sisters.
It should be recalled that no specific term exists in Hebrew and Aramaic to express the word "cousin", and that the terms "brother" and "sister", therefore had a far broader meaning which included several degrees of relationship. In fact, the phrase "brothers of Jesus" indicates "the children" of a Mary who was a disciple of Christ (cf Mt 27:56) and who is significantly described as "the other Mary" (Mt 28:1). "They are close relations of Jesus, according to an Old Testament expression" (CCC, 500).
Mary Most Holy is thus the "ever virgin". Her prerogative is the consequence of her divine motherhood which totally consecrated her to Christ's mission of redemption.
(General Audience, August 28, 1996)