All 3 Uses of

brethren
in
Fearless

Then, after a punishing ocean swim, they would be presented with their Trident and deemed SEALs by their brethren.†
p. 127.1 *

The brethren of an evil ideology that spawned the terrorist attacks of September 11, 2001 claiming almost 3,000 innocent lives.... While the American media strives daily to erase the memory of the terrorist attacks of September 11, 2001, and paint this war as an unjust occupation of a sovereign nation, men like Mike and Nate are out here hunting down and destroying the enemies of the very freedom that allows our media to try and discredit us.†
p. 228.4

Adam returned to Hot Springs on March 23, met by the pallbearers he'd named in his CACO form: friends from his youth, Jeff Buschmann, Heath Vance, and Sean Merriott; his brother and brother-in-law, Shawn Brown and Jeremy Atkinson; and his SEAL brethren, Dave Cain, Austin Michaels, and Christian Taylor.†
p. 293.2

Definition:

brothers or members of the same community or group -- especially male members of a religious order