Definition from Wiktionary, the free dictionary
co-brother (plural co-brothers), also (in sense 1) co-brethren
- colleague ("brother" in the same craft, occupation, or society)
- My co-brethren of the quill.
1900, Arthur Francis Leach, Report on the manuscripts of the corporation of Beverley, page 91:
- At the death of the co-sisters of the Guild, every co-brother of the Guild to be present
- (India) One's wife's sister's husband.
2007, Sethi, A Handbook Of Standard English And Indian Usage: Vocabulary And Grammar, page 55:
- If your wife doesn't have a sister, you just can't have a co-brother.