Gen 29:12 reads as follows:
And Jacob told Rachel that he was her father's brother, and that he was Rebekah's son; and she ran and told her father. (KJV)
The KJV follows the Hebrew rather literally, translating אָח as "brother" while modern translations, in light of the family relationship between Jacob and Rachel's father (Laban).
The LXX translates אָח as αδελφος, though English translations of the LXX does not translate the term as "brother" but instead use terms such as "relative" or "near-kinsman" (e.g., Brenton; NETS) due to the context.
Some Catholic apologists have appealed to the elasticity of αδελφος in the LXX as support for their understanding of the term when uses of the brothers of Jesus. However, as I document in my book, Behold the Mother of My Lord: Towards a Mormon Mariology (2017), as has others, including Eric Svendsen (Who is My Mother? The Role and Status of the Mother of Jesus in the New Testament and Roman Catholicism [2001]) such apologists are guilty of “semantic obsolescence” as there is no unequivocal use of αδελφος and its feminine equivalent αδελφη having such an active meaning in the New Testament and contemporary literature.
Furthermore, Josephus is a witness against understanding αδελφος, when used of family members, in such an elastic manner as required by Catholic dogmatic theology. While some Catholics have abused Josephus to support such, including most recently Brant Pitre and his abuse of Jewish Antiquities (see αδελφος in Josephus' Jewish Wars, 6:356-57), how Josephus understood Gen 29:12 is instructive.
In Antiquities of the Jews 1:290, we read Josephus' more precise paraphrase of Gen 29:12 where he clarifies the Hebrew (and LXX):
For my mother Rebekah was sister to Laban your father, both by the same father and mother; I therefore and you are first cousins; and I am now come to greet you, and to renew that affinity which is proper between us.
The Greek term translated “first cousins” is ανεψιος (“cousin”) showing that Josephus did know the distinction between αδελφος and ανεψιος, and did not treat αδελφος in a manner Catholic apologists and interpreters do. Such is yet another blow against the perpetual virginity of Mary.