All, and I mean all, scholars of the bible and interpreters, used brother to mean brother from Genesis to Revelation.
The Hebrew words "
אחים" ('âchiem) and "
אָח" ('âch) and their Koine Greek equivalents "
ἀδελφός" (adelphos) and "
ἀδελφοὶ" (adelphoi) share the definitions "brother" and "kinsmen". The word "
brother" can refer to other types of kinsmen as well, not only siblings. See examples below.
The Septuagint translators substituted the Hebrew words "
אחים" ('âchiem) in Gen. 13:8 with the Greek equivalent "
ἀδελφοὶ" (adelphoi) and "
אָח" ('âch) in Gen 14:14 with the Greek equivalent "
ἀδελφός" (adelphos) to show
kinship between Abraham and Lot, which lineage shows were that of
uncle and
nephew: "εἶπεν δὲ Αβραμ τῷ Λωτ Μὴ ἔστω μάχη ἀνὰ μέσον ἐμοῦ καὶ σοῦ καὶ ἀνὰ μέσον τῶν ποιμένων μου καὶ ἀνὰ μέσον τῶν ποιμένων σου. ὅτι ἄνθρωποι
ἀδελφοὶ ἡμεῖς ἐσμεν." (Gen. 13:8) and "ἀκούσας δὲ Αβραμ ὅτι ᾐχμαλώτευται Λωτ ὁ
ἀδελφὸς αὐτοῦ, ἠρίθμησεν τοὺς ἰδίους οἰκογενεῖς αὐτοῦ, τριακοσίους δέκα καὶ ὀκτώ, καὶ κατεδίωξεν ὀπίσω αὐτῶν ἕως Δαν." (Gen. 14:14)
The Septuagint translators substituted the Hebrew word "
אָח" ('âch) in Gen. 29:15 with the Greek equivalent "
ἀδελφός" (adelphos) to show
kinship between Jacob and Laban, which lineage shows were that of
uncle and
nephew: "Εἶπε δὲ Λάβαν τῷ ᾿Ιακώβ· ὅτι γὰρ
ἀδελφός μου εἶ, οὐ δουλεύσεις μοι δωρεάν· ἀπάγγειλόν μοι, τίς ὁ μισθός σου ἐστί;" (Gen. 29:15
The Septuagint translators substituted the Hebrew word "
אחים" ('âchiem) in 1 Chr. 23:21–22 with the Greek equivalent "
ἀδελφοὶ" (adelphoi) to show it was
kin the daughters of Eleazar married, which lineage shows was their
cousins: "καὶ ἀπέθανεν Ελεαζαρ, καὶ οὐκ ἦσαν αὐτῷ υἱοὶ ἀλλ᾽ ἢ θυγατέρες, καὶ ἔλαβον αὐτὰς υἱοὶ Κις
ἀδελφοὶ αὐτῶν." (1 Chr. 23:22)
In the New Testamemt in Matt. 13:55/Mk. 6:3, Joseph, Simon, James, and Judas (Jude/Thaddeus) were called Jesus's "
ἀδελφοὶ" (adelphoi). In the opening post is evidence that shows they were the sons of Joseph's brother, Alphaeus, which makes those kinsmen of Jesus His cousins.
So, all the examples above show you're wrong to say that the word "
brother" has
only been used to refer to siblings from Genesis to Revelation.