Many people ask about the alleged siblings of Jesus mentioned in the New Testament (Matthew 12:46, 13:55; Mark 3:31–32, 6:3; Luke 8:19–20; John 2:12, 7:3–5; Acts 1:14; 1 Corinthians 9:5). These passages refer to Mary and the “brethren” or the “brothers and sisters” of Jesus. First, we go to the original Greek text of these citations.
The word used to describe these siblings, as some describe them, is adelphoi. This is the plural form of adelphos, which can mean “brother,” but it is also used to describe more distant relations, like cousins, uncles, and nephews. Any relative or relation, from brother to cousin, can be called an adelphos since ancient Hebrew had no precise distinctions as we do today. Proof of this is in the Bible itself.
Genesis 11:27 (“Terah begat Abram, Nahor, and Haran; and Haran begat Lot.”) and 11:31 (“and Lot the son of Haran”) and 14:12 (“and they took Lot, Abram’s brother’s son”) make it clear that Abram and Haran are brothers and Lot is the son of Haran, which makes Abram his uncle. Lot is therefore the nephew of Abram. Yet the only way ancient Hebrew could describe the relationship between Abram and Lot was to either say it in a very complicated way, as in Genesis 12:5 where it says “and Lot (Abram’s) brother’s son” or it can use an inclusive word like adelphos in Greek or (ach) in Hebrew. This is done in Genesis 14:16: “his brother Lot.” If Lot was the son of Haran, the brother of Abram, then how can Abram call Lot his brother in 14:16? This only makes sense if the words used in Hebrew and Greek are not exclusively restricted to one form of relationship. Adelphos is the Greek word used in the Septuagint, the Greek translation of the Old Testament, and “brother” is used in the English King James Version of the Bible in Genesis 14:14, 16. Obviously, without having a word like “uncle” or “nephew,” the same word used for “brother” is also used for these extended relationships. Hence, when we read in the New Testament about the adelphoi of Jesus, they can be the same kind of relatives Abram and Lot were to each other. Jesus did have one cousin for certain, John the Baptist.
Joseph could have had siblings with children, who would have been his nieces and nephews and legal cousins to Jesus. Mary could have had siblings or cousins with children the same age as Jesus, too. If a brother of Joseph died leaving children to be raised, Joseph would have taken them into his home and Jesus would have been raised with his cousins, who would have been called adelphoi in Greek and, in English, that particular word is translated as “brethren.”
Further proof of the nonexistence of other children of Mary and Joseph: where are they at the Crucifixion? John 19:26–27 has Jesus giving His mother Mary to John and vice versa. Had there been any actual siblings, they would have been there with Mary at Calvary and there would have been no need to entrust his blessed mother to the care of John the Beloved Disciple.