This question arises because the gospels refer to the “brothers” and “sisters” of our Lord. In the New American Bible’s English translation of the Gospel of St. Mark, we do indeed read about the crowd asking, “Isn’t this the carpenter, the son of Mary, a brother of James and Joses and Judas and Simon? Aren’t his sisters our neighbors here?” (Mark 6:3). A similar reference occurs earlier in Mark 3:31 — “His mother and brothers arrived….”
The problem emerges in understanding the meaning of the word brother. In the original text of the gospel, we find the Greek word adelphos, meaning brothers, used. However, adelphos does not just mean blood brothers born of the same parents. Rather, adelphos was used to describe brothers not born of the same parents, like a half-brother or step-brother. The word also described other relationships like cousins, nephews, uncles, etc. For example in Genesis 13:8 and 14:14-16, the word adelphos was used to describe the relationship between Abraham and Lot; however, these two men did not share a blood brother relationship, but one of uncle and nephew. Another instance is that of Laban, who was an adelphos to Jacob, not as a brother, but as an uncle. (In the New American translation, “kinsman” or “relative” will be used in these Old Testament cases; I do not know why this is not true in the English translation of the gospel.)
The same meanings are true for the word sister in Greek. For example, in I Chronicles 23:21-22, the sons of Kish married their “sisters,” a literal translation of the text, but in reality they married their cousins.
Actually the confusion originates in Hebrew and Aramaic, the languages of most of the original Old Testament texts and of Christ. In these languages, no special word existed for cousin, nephew or aunt, half-brother or half-sister, or step-brother or step-sister; so they used the word brother or a circumlocution, such as in the case of a cousin, “the son of the brother of my father.” When the Old Testament was translated into Greek and the New Testament written in Greek, the word adelphos was used to capture all of these meanings for male relatives. So in each instance, we must examine the context in which the title is used. In all, the confusion arises in English because of the lack of distinct terms for relatives in the Hebrew and Aramaic, and the usage of the Greek adelphos to signify all of these relations.
Nevertheless, other gospel passages clarify these relationships. James and Joses were the sons of Mary of Cleophas (Mark 15:40). Mary of Cleophas is described in the Gospel of John as our Blessed Mother’s “sister” (John 20:25); obviously, she must have been a cousin, and James and Joses thereby cousins of our Lord. Judas was the son of James (not either of the apostles) (Luke 6:16). James the lesser was the son of Alphaeus (Luke 6:15). James the greater and John were the sons of Zebedee with a mother other than our Blessed Mother Mary (Matthew 20:20ff).
The gospels are also very clear that Mary was a virgin at the time she conceived Jesus through the power of the Holy Spirit (cf. Matthew 1:18-25, Luke 1:26-38). Remember when the Archangel Gabriel announced to Mary God’s plan, she responded, “How can this be since I do not know man?” After the birth of our Lord, although the gospels do not give us many details of His childhood, no mention is made of Mary and Joseph ever having other children. Never does it refer to the “sons of Mary” or “a son of Mary,” but only the son of Mary.
This point is again corroborated at the crucifixion scene: Before He dies, our Lord says to Mary, “Woman, there is your son,” and then to St. John, who is definitely not a blood brother, “There is your mother.” According to Jewish law, the oldest son had the responsibility of caring for the widowed mother, and that responsibility would pass to the next oldest if anything happened to the first born son. By this time, St. Joseph had died. Since Jesus, the first born, had no “blood brother,” He entrusted Mary to the care of St. John, the Beloved Disciple.
Interestingly, the Orthodox Churches solve this problem over brothers and sisters by speculating that St. Joseph was a widower who had other children before he married Mary. These brothers and sisters would really then be half-brothers and half-sisters. Perhaps this notion is why St. Joseph sometimes appears elderly in paintings.
Actually, this whole confusion is not new. About 380, Helvidius suggested that the “brethren” were the children born of Mary and Joseph after Jesus. St. Jerome declared this as a “novel, wicked, and daring affront to the faith of the whole world.” In his On the Perpetual Virginity of the Blessed Mary, St. Jerome used both Scripture and the fathers like Saints Ignatius, Polycarp, Irenaeus and Justin Martyr to refute Helvidius. Later, the First Lateran Council (649) definitively declared that Mary was “ever virgin and immaculate.” Therefore, as Catholics, based on Sacred Scripture and Tradition, we do not believe that Mary and Joseph had other children and consequently that Jesus had blood brothers and sisters.