Hi
Of the four “brethren” who are named in the Gospels, consider, for the sake of argument, only James. Similar reasoning can be used for the other three. We know that James the younger’s mother was named Mary. Look at the descriptions of the women standing beneath the cross: “among whom were Mary Magdalene, and Mary the mother of James and Joseph, and the mother of the sons of Zebedee” (Matt. 27:56); “There were also women looking on from afar, among whom were Mary Magdalene, and Mary the mother of James the younger and of Joses, and Salome” (Mark 15:40).
Then look at what John says: “But standing by the cross of Jesus were his mother, and his mother’s sister, Mary the wife of Clopas, and Mary Magdalene” (John 19:25). If we compare these parallel accounts of the scene of the crucifixion, we see that the mother of James and Joseph must be the wife of Clopas. So far, so good.
An argument against this, though, is that James is elsewhere (Matt. 10:3) described as the son of Alphaeus, which would mean this Mary, whoever she was, was the wife of both Clopas and Alphaeus. But Alphaeus and Clopas are the same person, since the Aramaic name for Alphaeus could be rendered in Greek either as Alphaeus or as Clopas. Another possibility is that Alphaeus took a Greek name similar to his Jewish name, the way that Saul took the name Paul.
So it’s probable that James the younger is the son of Mary and Clopas. The second-century historian Hegesippus explains that Clopas was the brother of Joseph, the foster-father of Jesus. James would thus be Joseph’s nephew and a cousin of Jesus, who was Joseph’s putative son.
Hope this helps, I’m a bit busy so have to hurry with my answer, let me know if this is clear.