In Rabbi Yeshua ’s time neither Hebrew nor Aramaic had a word meaning cousin. The Hebrew and Aramaic word ah was used for many relationships such as brother, cousin or nephew. Lot was Abraham’s nephew, the son of Abraham’s brother Haran Gen 11:27, but their relationship, described by the Hebrew word ah, Gen 13:8 is translated as “brothers” in the New International Version, “brethren” in the Masoretic (Jewish) Text, the Douay-Rheims Version and the King James Version, and “kinsmen” in the Revised Standard Version and the New American Bible. Laban was Jacob’s uncle, the brother of Rebekah (Jacob’s mother) Gen 27:43, but we find Laban’s relationship to Jacob translated as, “brother” Gen 29:15 in the Masoretic Text, the Douay-Rheims, and the King James, “relative” in the New American Bible and the New International Version, and “kinsman” in the Revised Standard Version.
After David conquered the Amalekites, some of the men who fought beside him tried to refuse sharing what they had taken with two hundred others who had been too exhausted to cross the Wadi Besor and participate. David replied, “You shall not do so, my brothers” 1 Sam 30:23. The Book of Chronicles speaks of, “…the Hebronites, Hashabiah and his brethren, one thousand seven hundred men of ability…” 1 Chron 26:30, and “King David appointed him and his brethren, two thousand seven hundred men of ability…” 1 Chron 26:32. These thousands were not children of the same mother.
The apostles and evangelists spoke Aramaic every day because it was the common language of Palestine. They wrote the New Testament in Greek because it was spoken by most Christians outside Palestine. Greek had words for cousin and other kinds of relatives, but the New Testament uses the Greek word adelphos, which means a real brother. The inspired writers of the Gospels and Epistles, especially St. Paul, often had Aramaic in mind when they wrote Greek. For example, Aramaic lacked degrees of comparison, such as good, better, best; it used opposites such as love and hate to express “more” or “less.” The difference was clear from context. Luke, writing in Greek, quoted Our Lord, “If any one comes to Me and does not hate his own father and mother and wife and children and brothers and sisters, yes, and even his own life, he cannot be My disciple” Lk 14:26. Rabbi Yeshua meant we are to love him even more than we love our earthly family.
Some Christians read that Rabbi Yeshua had brothers and sisters, and ask how Mary could have remained a virgin all her life. The Torah command, “Honor your father and your mother,” Deut 5:16 meant that any other children to whom Mary had given birth would protect and take care of her. Rabbi Yeshua affirmed from the cross that Mary had no other children by giving her John as a protecting son. We know that James, the “brother of the Lord” Gal 1:19 was alive at the time, so Mary certainly did not give birth to him.
When the apostle Paul and his companions were in Pamphylia, the synagogue officials sent word to them, “Brethren, if you have any word of exhortation for the people, say it” Acts 13:15. When Peter stood before the Council of Jerusalem, debating whether Torah was mandatory for Christians, he addressed the apostles and presbyters, “Brethren, you know that in the early days…” Acts 15:7. And James responded, “Brethren, listen to me” Acts 15:13. Paul addressed the Sanhedrin, “Brethren, I have lived be-fore God in all good conscience up to this day” Acts 23:1.
Paul’s epistles to the Christians in Rome, Corinth, Galatia, Philippi, Colossae, and Thessalonica refer constantly to his brothers and sisters in Rabbi Yeshua, as does the epistle to the Hebrews. James and John use the term often as well. The Catholic Church still calls its monks and friars, “brother” and its nuns and sisters, “sister.” Many Protestant and Bible Christian preachers proclaim from the pulpit, “Brothers and sisters, repent!” In all these cases, both ancient and modern, brother does not refer to children of the same mother.
The above from Second Exodus, p. 54-55.
Also see Brethren of the Lord