The Grammarphobia Blog

In-laws and other strangers

Q: I teach English as a second language and this question came up in class: John and Frank are brothers. John is married to Jennifer. Frank is married to Francine. Are Jennifer and Francine sisters-in-law? I think they are, but the ESL students insist they aren’t in other languages.

A: Yes, women who are married to brothers are sisters-in-law to each other.

The Oxford English Dictionary, Merriam-Webster’s Collegiate Dictionary (11th ed.), and The American Heritage Dictionary of the English Language (4th ed.) give three definitions of “sister in law”:

(1) The sister of one’s spouse.

(2) The wife of one’s brother.

(3) The wife of the brother of one’s spouse.

I think the confusion here is over the English term “in-law,” an idiomatic phrase that indicates someone is related by marriage, not blood.

Your students apparently think the expression “sisters-in-law” refers to legal (that is, actual or birth) sisters. It doesn’t. Here’s how the OED defines “in-law”:

“A phrase appended to names of relationship, as father, mother, brother, sister, son, etc., to indicate that the relationship is not by nature, but in the eye of the Canon Law, with reference to the degrees of affinity within which marriage is prohibited.”

Other languages have terms to describe such a relationship, though the wording may not include the legal terminology that English uses. For example, a sister-in-law is a belle-soeur in French, a cuñada in Spanish, and a cognata in Italian.

Interestingly, the term “in-law” was once used in English to describe relationships that are now referred to with the term “step.” So, the expression “sister-in-law” once also meant stepsister. The OED says this sense “though still locally or vulgarly current, is now generally considered a misuse.”

For what it’s worth, the first published reference in the OED for “sister-in-law” dates from the early 1400s, more than a century after the first appearance of “brother-in-law.”

I’ve gone on a bit, but I hope this helps.

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