The word(s) "brethren" appears 21 time(s) in 21 verse(s) in Quran in Muhammad Pickthal translation.

(21) Hast thou not observed those who are hypocrites, (how) they tell their brethren who disbelieve among the People of the Scripture: If ye are driven out, we surely will go out with you, and we will never obey anyone against you, and if ye are attacked we verily will help you. And Allah beareth witness that they verily are liars.