sister-in-law

American  

[sis-ter-in-law] / ˈsɪs tər ɪnˌlɔ /

noun

plural

sisters-in-law

1. the sister of one's spouse.

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

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

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sister-in-law British  

noun

1. the sister of one's husband or wife

2. the wife of one's brother

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Etymology

Origin of sister-in-law

First recorded in 1400–50; late Middle English syster in lawe; sister, in, law ¹; 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

Example Sentences

Jamshidi has family members, including his brother—who worked in a high-risk post as a security guard for the U.S. consulate in Herat, Afghanistan, for years—his sister-in-law, niece and nephew stuck in Camp As Sayliyah.

From The Wall Street Journal • Apr. 7, 2026

The employee was his sister-in-law, the person confirmed.

From Los Angeles Times • Feb. 18, 2026

My sister-in-law Eva Rose, who also lives nearby and also came to the intersection separately, described to me the moment the agents’ anger was unleashed a second time as she arrived 15 minutes later.

From Slate • Jan. 25, 2026

It’s the exact opposite of what you truly want, but your husband has allowed his brother and sister-in-law to coerce and/or guilt-trip your husband into an unwelcome financial arrangement.

From MarketWatch • Jan. 21, 2026

Dorothy’s own sister-in-law had moved to Washington to take a job in the War Department.

From "Hidden Figures" by Margot Lee Shetterly

Related Words

• aunt
• cousin
• father
• folk
• mother
• niece
• sibling
• uncle