Yes, BRETHREN is a Scrabble word!

Letter values in Scrabble US: B 3, R 1, E 1, T 1, H 4, R 1, E 1, N 1.

Is it a valid word?

BRETHREN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(plural) the lay members of a male religious order
a male with the same parents as someone else
a male person who is a fellow member (of a fraternity or religion or other group)
a close friend who accompanies his buddies in their activities
used as a term of address for those male persons engaged in the same movement
(Roman Catholic Church) a title given to a monk and used as form of address

Also: blood brother.