Several times in the New Testament are mentioned the brethren of Jesus. The use of the term “brethren” while frequent among Christians today when referring to “brother” Christians is rather used in the New Testament to refer to actual relatives of Jesus. Were they really His brothers?

Actually, there are four brothers and several sisters mentioned, raising the question, “Were they the children of Mary by Joseph?” Some theologians think so and that would be the simplest answer and would make them half siblings of Jesus. That is, they were children of Joseph and Mary after Jesus. 

An early view is that they were cousins of Jesus, that is, they were children of Mary’s sister and of Clopas. This view also holds that three of the brothers were among the 12 apostles.

My own view and that called the Epiphanian view is that they were children of Joseph by a previous marriage and that Mary was truly a virgin all her life. The reasons for this view are as follows:

1. It is the oldest viewpoint, being current in Palestine in the 2^nd century.
2. If Mary had a large family of her own, the tradition of her perpetual virginity could never have gotten traction which it did and still has, especially in the Catholic Church.
3. Jesus on the cross would not have committed his mother’s care to John if she had four living sons.

The Epiphanian view still is held in the Eastern Orthodox church.

A fair question is raised by the absence of any of them in the visit by Jesus at age 12 to the Temple in Jerusalem. But being older, they might have stayed at home or attended the feast by themselves.

His brethren were not believers during Jesus’ lifetime on earth but it does appear from Acts 1;14 and ICor 15;7 that they were converted after the resurrection of Jesus from the dead.