Plural of “brother-in-law”
The plural of brother-in-law is brothers-in-law (some dictionaries also accept brother-in-laws).
Why
"Brother-in-law" is a French-influenced compound built on the pattern "brother" plus "in-law," where "in-law" functions as a modifying phrase describing the relationship's legal, rather than blood, basis. Because "brother" is the true head noun of the compound, being the actual person the word names, and "in-law" simply specifies what kind of brother he is, the plural -s attaches to "brother," giving "brothers-in-law" rather than "brother-in-laws."
This hyphenated structure entered English kinship vocabulary in the medieval period, when French legal and ecclesiastical terms — including the underlying concept of a relative connected "by law" rather than by blood — reshaped a good deal of how English names family members, a legacy of the Norman Conquest working its way into everyday household vocabulary. Every relative connected through marriage rather than blood gets built the identical way in English, which is why "brother-in-law" behaves as one member of a small, tightly consistent compound family rather than an isolated oddity.
Usage note
Careful, formal writing — wills, inheritance documents, anything touching family law — sticks with "brothers-in-law" as the only accepted plural. You'll hear "brother-in-laws" often enough in relaxed conversation that nobody blinks, but edited prose treats it as a slip, so it's worth avoiding outside the most casual contexts.
The word covers three genuinely distinct relationships depending on family structure, a spouse's brother, a sibling's husband, or the husband of a spouse's sibling, all using the identical term and identical plural regardless of which specific relationship is meant, a real source of occasional ambiguity resolved only by context. Family law and inheritance documents sometimes need to specify exactly which kind of "brother-in-law" relationship is meant, since inheritance rights and legal obligations can differ depending on whether the relationship traces through marriage to a sibling or marriage to a spouse's sibling. In many cultures, brothers-in-law take on specific social roles and obligations distinct from those of blood brothers, sometimes including responsibilities around hosting, financial support, or mediating family disputes, roles that vary considerably across different cultural and regional traditions even though the English term itself stays the same.
Common real mistakes
The most frequent mistake is pluralizing at the very end, "brother-in-laws," treating the whole hyphenated compound as if it were one simple word rather than recognizing "brother" as the head noun carrying the plural. A related error is dropping the hyphens entirely in writing ("brother in laws"), which compounds the confusion about where the plural marker actually belongs.
The wider pattern
Grammatically, the plural marker in "brothers-in-law" lands on "brother" and stays off "law" for the same reason it lands on "mother," "sister," "son," and "daughter" in their own in-law compounds: "law" is never the head noun here, just a fixed descriptive tail, so the actual person being counted — the blood relative at the front of the phrase — is where English attaches the -s.
Examples
- Both of her brothers-in-law attended the wedding.
- He borrowed a truck from one of his brothers-in-law.
- The family reunion brought together brothers-in-law from three different states.
- She gets along well with all of her brothers-in-law.
- Two brothers-in-law started a small business together after college.
- His brothers-in-law helped move furniture into the new house.
- The will divided the estate equally among the surviving brothers-in-law.
- Her brothers-in-law surprised her with a birthday party.
- The holiday dinner seated brothers-in-law at opposite ends of the table.
- Both brothers-in-law coached youth soccer in the same league.
- Her three brothers-in-law all live within an hour of each other.
- The reunion photo included brothers-in-law from both sides of the family.
- He and his brothers-in-law spent the weekend fixing the fence together.
- The two brothers-in-law argued good-naturedly about the family recipe.
- She relied on her brothers-in-law during the difficult move.
- Their brothers-in-law arrived early to help set up chairs.