The claim that “brother” (a·del·phos′) here means “cousin” (a·ne·psi·os′) is a theoretical contention, the invention of which is credited to Jerome, and dates back no earlier than 383 C.E.   

Not only does Jerome fail to cite any support for his newborn hypothesis but in later writings he wavers in his opinions and even expresses misgivings about his “cousin theory.” J. B. Lightfoot states that “St Jerome pleaded no traditional authority for his theory, and that therefore the evidence in its favour is to be sought in Scripture alone. I have examined the scriptural evidence, and the . . . combination of difficulties . . . more than counterbalances these secondary arguments in its favour, and in fact must lead to its rejection.”—St. Paul’s Epistle to the Galatians, London, 1874, p. 258.