[ sis-ter-in-law ]
/ ˈsɪs tər ɪnˌlɔ /
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noun, plural sis·ters-in-law.

the sister of one's spouse.
the wife of one's brother or sister.
the wife of one's spouse's sister or brother.



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Origin of sister-in-law

First recorded in 1400–50; late Middle English syster in lawe; see origin at sister, in, law1; from Middle English in-lawe “in law,” i.e., “a person within the regulation and protection of the law,” based on the prohibition by Roman civil law and, later, Christian canon law, of marriages within four degrees of consanguinity, i.e., up to and including first cousins
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

British Dictionary definitions for sister-in-law


noun plural sisters-in-law

the sister of one's husband or wife
the wife of one's brother
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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