St Jerome thought that the brothers of Jesus were first cousins. The Biblical texts do not indicate this since cousins have a separate set of parents of their own. What is recounted is that Mary was always going round with the brothers of Jesus, in other words playing the role of mother towards them and they sons towards her. After Joseph their father died, the brothers and sisters had no other parents apart from Mary to be attached to.
Christians have long disputed the Bible texts referring to the brothers of Jesus, and by extension to this argument the perpetual virginity of Mary.
A simple reading of the gospels, without any probing or calculation of ages, appears to indicate what constitutes the Protestant view: that Mary as a virgin conceived Jesus and gave birth to him in Bethlehem. Then after that she lived a normal marriage and had about eight other children with Joseph in Nazareth. Joseph died leaving Mary with nine children. If this were the case, Jesus was aged about 32 when Joseph died, and any younger children would be aged about 30, 28, 26, 24, 22, 20, 18 and 16.
In the Catholic view the brothers and sisters of Jesus are proposed as being Joseph’s children from a previous marriage. These children would all be much older than Jesus. By my calculations when Jesus was 32 they would be aged about 53, 51, 49, 47, 45, 43, 41 and 39.
In support of the Catholic view is the information that from the cross, Jesus gave his mother to the apostle John as a mother; and he to her as a son.
Let us remember that women lived in their father’s household, then they married and lived in their husband’s household, and if he died they lived in their son’s household. Women did not live alone. To be an abandoned widow or orphan was the worst thing ever.
If, when Jesus died, there had been eight younger siblings to look after, Mary would have returned to Nazareth to take charge of them and look after them.
If, however, the siblings were older, with the sisters already married and some of the sons such as James also already married, then she would have no obligation to look after them.
Did the brothers of Jesus have an obligation to look after Mary? The four brothers remained in Jerusalem after the crucifixion and resurrection, and they became pillars of the early church. It would be natural that one of the brothers would take Mary into his household in Jerusalem, but this did not happen. The Catholic argument goes that this did not happen because she was not their genetic mother, she had not given birth to any of them; they were Joseph’s sons.
Jesus himself had an obligation towards Mary his mother, to whom he was actually genetically related, and he passed the responsibility to John.
Thus, the balance of evidence from the New Testament shows that the brothers were not the sons of Mary; she had one child, not nine children. The Catholic view is borne out by the Bible.