/ Thesaurus / B Synonyms of brother in law Find synonyms for: Noun1. brother-in-law, in-law, relative-in-lawusage: a brother by marriageWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: sumptuous Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language