sister-in-law

From Middle English suster-in-lawe; equivalent to sister +‎ in +‎ law.

sister-in-law (plural sisters-in-law)

1. A female relative of one's generation, separated by one degree of marriage:
   1. The sister of one's spouse.
   2. The wife of one's sibling.
2. (less common) Co-sister-in-law: The wife of one's sibling-in-law.
   1. The wife of the sibling of one's spouse.
   2. The sister of the spouse of one's sibling.

The plural sister-in-laws is occasionally seen, but this is considered incorrect by most sources; see, for example, , despite being a common construction in American English.

singular sister-in-law

• 1753, John Pringle, An Account of several Persons seized with the Goal-Fever, working in Newgate, in Philosophical Transactions, Vol. 48

  Some time after this, Mr. Breach the apothecary informed us, that he was again employ’d in Thomas Wilmot’s family; for that Elizabeth Marſhall, his ſiſter-in-law, after nurſing his wife, was taken ill of the ſame kind of fever, and deſired our aſſiſtance. This perſon we found in the ſame bed, and in the ſame condition, in which we had ſeen her ſiſter ſome time before...

plural form sisters-in-law

• 1874, Tseen-Han-shoo, History of the Heung-Noo in Their Relations with China, in The Journal of the Anthropological Institute of Great Britain and Ireland, Vol. 3

  Chang Hwuy, the master controller, went with Woo-sun troops to the right Luh-le’s court, captured the Shen-yu’s paternal relatives, sisters-in-law, ...

plural form sister-in-laws

• 1936, Margaret Mitchell, Gone with the Wind

  I have two sister-in-laws in Charleston and I know.

• half sister-in-law
• co-sister-in-law

• brother-in-law
• daughter-in-law
• father-in-law
• mother-in-law
• son-in-law