An Etymological Dictionary of the German Language/Bruder
Bruder, m., ‘brother, friar,’ from the equiv. MidHG. bruoder, OHG. bruodar; comp. Goth. brôþar, AS. brôþor, E. brother, Du. broeder, OSax. brôthar. Inherited, like most words denoting kinship, from the period when all the Aryans formed only one tribe, without any difference of dialect; the degrees of relationship (comp. Oheim, Vetter, Vase) at that period, which is separated by more than three thousand years from our era, were very fully developed. The primit. form of the word Bruder was bhrãtô(r), nom. plur. bkrâtores; this is attested, according to the usual laws of sound, both by Goth.-Teut. brôþar and Lat. frâter, Gr. φράτηρ, OInd. bhrâtar-, OSlov. bratrŭ; all these worlds retain the old primary meaning, but in Gr. the word has assumed a political signification.