Brethren

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Old English: archaic plural of brother.

wiktionary

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From Early Modern English brethren, plural of brother, from Middle English brethren, from Middle English brethere, brether + -en(plural ending). Ultimately from Old English brōþor, brōþru(“brothers, brethren”), influenced by Old English brēþer, dative singular of brōþor(“brother”). Equivalent to brother +‎ -enpl. Compare German Brüder(“brothers, brethren”). More at brother. The vowel change (from o to e) is called umlaut.

etymonline

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brethren (n.)

alternative plural of brother (q.v.); predominant c. 1200-1600s, but surviving only in religious usage and not now used in reference to male children of the same parents. The title was used by primitive Christians (Acts xviii, etc.) and so was taken by various Protestant sects, such as the Dunkers.