When the NT was written, the majority of Christians were Greek speaking Jews who would have been familiar with the Semitic idiomatic usage of the words adelphos and adelphe. We read in the Septuagint, the Greek translation of the Hebrew OT:
*υἱοὶ Μεραρί· Μοολὶ καὶ Μουσί. υἱοὶ Μοολί· ᾿Ελεάζαρ καὶ Κίς. καὶ ἀπέθανεν ᾿Ελεάζαρ, καὶ οὐκ ἦσαν αὐτῷ υἱοί, ἀλλ᾿ ἢ θυγατέρες, καὶ ἔλαβον αὐτὰς υἱοὶ Κὶς ἀδελφοὶ αὐτῶν. υἱοὶ Μουσί· Μοολὶ καὶ ᾿Εδὲρ καὶ ᾿Ιαριμώθ, τρεῖς.
The sons of Merari; Mooli, and Musi: the sons of Mooli; Eleazar, and Kis. And Eleazar died, and he had no sons, but daughters: and the sons of Kis, their** brethren** (brothers), took them. The sons of Musi; Mooli, and Eder, and Jarimoth, three.
1 Chronicles 23, 21-23
The Greek word for “brother” (adelphos/ἀδελφοὶ : of the same womb) is used in sacred Scripture in reference to cousins (ἀνεψιός/anepsios) in keeping with Hebrew parlance. The daughters of Eleazar married the sons of his brother Kish. There’s a Greek word for cousin, but it isn’t used.
By the way, Matthew originally wrote his gospel in Hebrew.
In the New Testament Greek when Jesus’ brothers and sisters are mentioned in verses like John 2:12 forms of the words adelphos and adelphe are used. These words always mean brothers or sisters - including spiritual brothers and sisters of the same faith.
The Gospel of Mark was originally written in Greek, so you’re mistaken that the word adelphos “always” means uterine brothers. Observe Mark 6:17-18*: ‘For Herod himself had given orders to have John arrested, and he had him bound and put in prison. He did this because of Herodias, his brother Philip’s wife, whom he had married. For John had been saying to Herod, “It is not lawful for you to have your brother’s wife.”’* Are we to presume that Herod Antipas and Philip were uterine brothers? Certainly not! Herod Antipas was the son of Mariamne the Hasonean, Herod the Great’s second wife. Philip the Tetrarch (Herod Philip 1) was the son of Cleopatra of Jerusalem, Herod’s fifth wife. Thus the two men are half-brothers, having the same father but different mothers. Mark employs the Semitic idiomatic term because there is no single word for half-brother or step-brother in Hebrew and Aramaic - just as there are no Hebrew words for uncle, nephew, aunt, and niece. Unfortunately, Protestants overlook or choose to ignore this passage when citing Mark 6:3 (cf. Mt. 12:46-50) as a proof-text against the Catholic dogma of Mary Ever-Virgin. And they fail to notice or choose to ignore that these supposedly biological brothers and sisters of Jesus are actually never called children of Mary, though Jesus himself is explicitly referred to as a child of hers (Jn. 2:1; Acts 1:14).
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