A faithful reader sent a comment about how hyphenated plural nouns such as mother-in-laws and sister-in-laws drives her crazy. She actually said that hearing this grammar error sounds like fingernails on a chalkboard, and since we all know how that can make us grind our teeth, I’m ready to jump in there and silence the scraping once and for all.
It’s understandable that making the singular noun mother-in-law into a plural noun would result in many of us making the grammar error mother-in-laws since plurals of nouns are usually made by adding the -s to the END of the word, for example, mothers, hurricanes, aardvarks, and molecules. In this case, however, the plural is made by adding the -s to the FIRST word, not the last. Think of it this way: It’s the mothers, not the laws, that are plural. The correct plural of mother-in-law, then, is mothers-in-law.
The same is true for other hyphenated nouns such as doctors-in-residence, attorneys-at-law, and fathers-in-law.