I have a sister. What are her husband's parents to me (i.e. her parents in-law)?
Your sister is married, her husband is your brother-in-law, when referring to his parents you can say: my brother-in-law's parents
Google Books has 91 hits for brother-in-law's parents
As friends and family visited my sister with offers of food, money and emotional support.My brother-in-law's parents, brother, sister, and extended family came from Brooklyn, Pennsylvania, Long Island and Florida.
Your sister is married and her husband's parents are her in-laws. Her husband's mother is her mother-in-law, while his father is her father-in-law. Consequently, you could also call them: my sister's parents-in-laws.
But astonishingly, Google Books has no results for sister's parents-in-laws.
Instead, the shortened in-laws is preferred. The expression: sister's in-laws is reported 363 times by Google Books.
My sister's in-laws did not live in Nanjing, and this created a housing problem. Most new couples could not get a housing unit from the government. They had to live with one or the other's parents for a while, usually the husband's.
Thirty Years in a Red House: A Memoir of Childhood and Youth in Communist China
All of the couple's siblings live here - as well as her sister's in-laws, his sister's in-laws, and many cousins. They are an extended family that would do any Midwestern town proud,...
Calling Arizona Home