co-brother

co- +‎ brother

co-brother (plural co-brothers or co-brethren)

1. 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

2. (India) One's spouse's sibling'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.

• confrère
• co-brother-in-law (in the more common sense only)

• brother-in-law

• co-sister