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I just noticed on Facebook that someone I know referred to his “sister in laws” [sic]. This needs fixing in a couple of ways.

First off, all in-laws are hyphenated — brother-in-law, mother-in-law, sister-in-law, etc.

Second, when you are referring to plural in-laws of the same kind, you do not make in-law plural; rather, it’s the relationship that gets the S added at the end. Some examples:

• My two mothers-in-law and my two fathers-in-law (yes, I had two of each at one point, as my husband’s parents were divorced and remarried)
• My four brothers-in-law
• My four sisters-in-law
• And, one day, way in the future, my two sons-in-law

The easiest way to remember how to make these in-law relationships plural is to remember that if you were referring siblings, you would say, “my two sisters” or “my two brothers.”