St Mark relates that when Jesus was teaching in the synagogue of Nazareth, the people said: “Is not this the carpenter, the son of Mary and brother of James and Joses and Judas and Simon, and are not his sisters here with us? (Mk 6:3). So yes, the Bible says Jesus had “brothers” and “sisters.” But they were not other children of Our Lady. This is clear from other passages.
On relating the crucifixion scene, Mark says: “There were also women looking on from afar, among whom were Mary Magdalene, and Mary the mother of James the younger and of Joses…” (Mk 15:40).
From this passage, it is clear that James and Joses were sons of a different Mary, since Mark would obviously not have referred to Our Lady as the mother of James and Joses. Mark mentions this other Mary two more times. At Jesus’ burial he calls her “Mary the mother of Joses” (Mk 15:47), and on the morning of the resurrection “Mary the mother of James” (Mk 16:1).
From this it is clear that James, Joses, Judas and Simon were sons of another Mary, not of Our Lady.
They were “brothers” of Jesus in the sense that the Greek word for brothers, adelphoi, could refer both to blood brothers and to cousins, or relatives. Jesus himself calls these others his cousins in the passage of Mark which we quoted earlier: “‘Is not this the carpenter, the son of Mary and brother of James and Joses and Judas and Simon, and are not his sisters here with us?’ … And Jesus said to them, ‘A prophet is not without honour, except in his own country, and among his own cousins (Greek syngeneusin) and in his own house.’” (Mk 6:3-4).
Another powerful reason for affirming that these men were not Jesus’ blood brothers is that, if they were, Jesus on the cross would have entrusted his mother to one of them, rather than to St John (cf. Jn 19:26-27). If Jesus entrusted Mary to St John, it is because she had no other children.
Do we know anything about the relationship between this other Mary and Jesus? Some have suggested she was the wife of St Joseph from a previous marriage. But this is impossible, since for Joseph to be a widower, his previous wife would have had to have died, but this other Mary was still alive.
The Christian historian Hegesippus (110-180 AD) answers the question for us.
He says that the brothers James and Simon, the first two bishops of Jerusalem, were the sons of Clopas and cousins of Jesus. We remember that St John relates that at Mount Calvary, “standing by the cross of Jesus were his mother, and his mother’s sister, Mary the wife of Clopas, and Mary Magdalene” (Jn 19:25).
Eusebius, in his Church History, quotes Hegesippus, relating that after the martyrdom of James, the apostles and disciples met to decide who was to succeed him: “They all with one consent pronounced Simon, the son of Clopas, of whom the Gospel also makes mention, to be worthy of the episcopal throne of that parish. He was a cousin, as they say, of the Saviour.
For Hegesippus records that Clopas was a brother of Joseph” (Church History, 3.11.1-2). That answers our question. Since James and Simon were the sons of Mary and Clopas, St Joseph’s brother, they were in fact first cousins of Jesus, as was widely known at the time.
Apart from the scriptural and historical evidence we have just seen, the most powerful reason why the church teaches that Jesus had no brothers or sisters is sacred Tradition. The early Christians knew Mary, Joseph and Jesus very well, and they all knew and passed on to others that Jesus was the only son of Mary.
Summing up, the Catechism of the Catholic Church teaches: “The church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, ‘brothers of Jesus’, are the sons of another Mary, a disciple of Christ, whom St Matthew significantly calls ‘the other Mary’ (cf. Mt 27:56; 28:1).
They are close relations of Jesus, according to an Old Testament expression (cf. Gen 13:8; 14:16; CCC 500).