brother (n.) Look up brother at Dictionary.com
Old English broþor, from Proto-Germanic *brothar (cognates: Old Norse broðir, Danish broder, Old Frisian brother, Dutch broeder, German Bruder, Gothic bróþar), from PIE root *bhrater (cognates: Sanskrit bhrátár-, Old Persian brata, Greek phratér, Latin frater, Old Irish brathir, Welsh brawd, Lithuanian broterelis, Old Prussian brati, Old Church Slavonic bratru, Czech bratr "brother").

A highly stable word across the Indo-European languages. In the few cases where other words provide the sense, it is where the cognate of brother had been applied widely to "member of a fraternity," or where there was need to distinguish "son of the same mother" and "son of the same father." E.g. Greek adelphos, probably originally an adjective with frater and meaning, specifically, "brother of the womb" or "brother by blood;" and Spanish hermano "brother," from Latin germanus "full brother." As a familiar term of address from one man to another, it is attested from 1912 in U.S. slang; the specific use among blacks is recorded from 1973.