In the Hebrew manner of speaking brother meant any relative from which you are not descended including cousins, nephews, etc. (A male relative from which you are descended is a father)
Gen. 14:14-16 – Lot is called Abraham’s brother even though he is his nephew.
Gen. 29:15 – Laban calls Jacob his brother even though he is his uncle.
2 Sam. 1:26 – David calls Jonathon his brother.
Duet. 23:7 – You shall not abhor an Edomite he is your brother.
1 Chron. 23:21-22 – Eleazar and Kish were sons of Mahli. Kish had sons but Eleazar had only daughters. They married their brethren the sons of Kish who were really their cousins.
There is no word for cousin in Hebrew or Aramaic which were the languages spoken by Christ and his Apostles. Since the New Testament writers were brought up using brother to mean cousin etc. they used the same idiom in writing the Gospels. The Greek Jews did the same in translating the Old Testament from Hebrew to Greek in the Septuagint.
Mk. 6:3 – Jesus had four brothers – James, Joses, Simon, and Jude.
Mk. 15:40 – At the cross were Mary Magdalene, and Mary the mother of James the Less and Joses.
John 19:25 – At the cross were Jesus’ mother, and Mary the wife of Cleophas, which is a Chaldean rendering of the Hebrew Alphaeus.
Acts 1:13 – James the son of Alphaeus and Jude the brother of James.
So Mary, the wife of Alphaeus, (Cleophas) is the mother of James, Joses, Jude, and by extension Simon.
Cleophas – Klopas – of Chaldean origin. (Corresponding to G256); an Isr. …Clopas.
G256 – Alphaios, of Hebrew origin, Alphoeus, an Isr.; Alpheus.
Acts 1:13 – Judas – Ioudas – Jude.
Mk. 6:3 – Jesus is referred to by the people as “the” son of Mary not “a” son of Mary. The others are here and elsewhere called Jesus’ brethren, but never are they called the sons of Mary and Joseph.
Jn. 7:3-5 – In Eastern societies the elder gave advice to the younger not the reverse. In this passage his brethren are giving him advice, which implies they are older.
Luke 2:41-51 – When Jesus is found in the Temple at the age of 12 there is no hint of his having siblings.
Ex. 13:2 – In Matthew 1:25 Jesus is called the firstborn son. As Exodus 13:2 shows the firstborn son was simply the one who opened the womb and does not imply other children.
John 19:26-27 – If Jesus had blood brothers why would he have given Mary to John who was clearly the son of Zebedee. In Hebrew society this just wouldn’t be done and would have been a tremendous insult to his family.
Although it is unnatural to remain a virgin in marriage, it is also unnatural for a virgin to give birth to the Son of God.
Tradition says that Mary was dedicated at a very early age as a Temple Virgin and that Joseph who was much older took her to be a guardian over her.