brethren n.

"male person in his relation to another person or other persons of either sex born of the same parents," Old English broþor, from Proto-Germanic *brothar (source also of Old Norse broðir, Danish broder, Old Frisian brother, Dutch broeder, Old High German bruodar, German Bruder, Gothic bróþar), from PIE root *bhrater-.

A stable word across the Indo-European languages (Sanskrit bhrátár-, Greek phratér, Latin frater, etc.). Hungarian barát is from Slavic; Turkish birader is from Persian.

Other words sometimes come to mean "brother," when the cognate of brother is widely to "member of a fraternity," or as an appellation of a monk (Italian fra, Portuguese frade, Old French frere), or where there was need to distinguish "son of the same mother" from "son of the same father." For example Greek adelphos, which probably originally was an adjective with phrater and meant, specifically, "brother of the womb" or "brother by blood," and became the main word as phrater became "one of the same tribe." Spanish hermano "brother" is from Latin germanus "full brother" (on both the father's and mother's side); Middle English also had brother-german in this sense.

The meaning "male person in relation to any other person of the same ancestry" in English is from late 14c. The sense of "member of a mendicant order" is from c. 1500. As a familiar term of address from one man to another, it is attested from 1912 in U.S. slang; the specific use among Black American is by 1973.

bhrāter-, Proto-Indo-European root meaning "brother." 

It forms all or part of: br'er; brethren; ‌‌brother; bully (n.); confrere; fraternal; fraternity; fraternize; fratricide; friar; friary; pal.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhrátár-, Old Persian brata, Greek phratér, Latin frater, Old Irish brathir, Welsh brawd, Lithuanian broterėlis, Old Prussian brati, Old Church Slavonic bratru, Czech bratr, Polish brat, Russian bratŭ, Kurdish bera; Old English broþor, Old Norse broðir, German Bruder, Gothic bróþar. 

also sistern, an occasional, and obsolete or archaic, plural of sister (n.), Middle English sustern, sustren (c. 1200), perhaps by influence of brethren or of Old English gesweosternu, gesustrenu, adjective forms of the word used as plural nouns.