There has been much discussion through the centuries as to the exact relationship of these siblings to Jesus.
Three principal views have been advanced about Jesus’ siblings:
(1) The view that they were Jesus' actual siblings, that is, half-brothers or sisters (step siblings because Jesus was a Son of God and Mary, not Joseph and Mary), and sons and daughters of Joseph and Mary (and therefore younger than Jesus).
(2) The view that they were His stepbrothers and sisters, that is, children of Joseph by a previous marriage (and thus all older than He and not His blood relatives at all).
(3) The view that they were the cousins of Jesus on the mother's side, according to some, or on Joseph's side, according to others.
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Those who hold the first view argue that this is the most natural way to understand the various references to these brothers; also that this is the most obvious intent of Matthew 1:25 and Luke 2:7.
Those who hold the second view argue that family ethics would not permit younger siblings to taunt or otherwise meddle with an older brother as Jesus' brothers taunted Him (see Mark 3:31; John 7:3-4). They point out further that the fact that Jesus left His mother in the care of the apostle John (John 19:26-27) rather than with one of His brothers strongly implies that Mary had no other children.
The view that these brothers were the cousins of Jesus on Joseph's side is based on pure conjecture. That they were cousins on Mary's side is based on the equation of "Mary, the wife of Clopas" with the sister of Mary (John 19:25; Mark 15:40), and the equation of "Clopas" with Alphaeus (Mark 3:18). Neither of these claims are provable from Scripture.
Jesus' siblings are mentioned as accompanying Jesus and his mother to Capernaum after the marriage at Cana (John 2:12). Later Mary and these brothers are recorded as seeking an audience with Jesus (Matthew 12:46-50; Mark 3:31-35; Luke 8:19-21). Toward the end of Jesus' ministry, His brethren are mentioned as urging Jesus to prove His Messiahship, which they themselves doubted (John 7:3-5). That they were later converted is clear, for they are described in Acts as uniting with the disciples and others in "prayer and supplication" prior to Pentecost (Acts 1:13-14). Paul implies that they were all married (1 Corinthians 9:5).
Many commentators hold that the author of the epistle of Jude, who identifies himself as the "brother of James," was one of these brothers (Jude 1). It is also generally believed that the leader of the church at Jerusalem was James the brother of Jesus, (see Acts 12:17; 15:13). This seems to be confirmed by Paul's reference to his visit to Jerusalem, in which he states that he saw only Peter, and "James, the Lord's brother" (Galatians 1:18-19).
No matter what kind of siblings they were, it is clear that none of His siblings chose to set up a dynasty to perpetuate Jesus’ power on earth after He returned to heaven. Instead, they pointed others toward Jesus and His saving power.