fraternal (adj.)

early 14c., "body of men associated by common interest," from Old French fraternité (12c.), from Latin fraternitatem (nominative fraternitas) "brotherhood," from fraternus "brotherly," from frater "brother," from PIE root *bhrater- "brother." Meaning "state or condition of being as brothers" is from late 15c. College Greek-letter organization sense is from 1777, first in reference to Phi Beta Kappa.

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. 

1610s, from fraternal + -ly (2).