Question: “People at my office often talk about their ‘brother-in-laws’ or ‘sister-in-laws,’ and it drives me crazy. Please write an article clearing up this nonsense. Hopefully some of them will see it.”
Answer: Sounds like this one has been wearing on you for a while. I’ll see what I can do.
Words like “brother-in-law,” “U-boat,” “not-for-profit,” etc., are known as hyphenated compound nouns. They can get a bit tricky at times; since there are often several words you have to deal with when deciding where to place an “s” to make their possessive form.
The rule in this case is to place the “s” after the primary noun: “brothers-in-law,” “U-boats,” “not-for-profits.” Why “brothers-in-law” and not “brother-in-laws?” Well, think: Are you saying you have multiple brothers (by law), or that you have a brother due to multiple laws? I’m guessing the former. The same logic holds for “sisters-in-law,” “parents-in-law,” etc.
Now, if we want to make a hyphenated compound noun possessive, things are very simple: Add an apostrophe and an “s.” “My brother-in-law’s boat is ready for the summer.” “The not-for-profit’s fundraiser should be a blast.” We don’t always get simple, standard rules in English, so enjoy this one and don’t make it any harder than it needs to be.
Just to be thorough: On the off-chance that you ever wanted to make a hyphenated compound noun plural and possessive, you just follow both rules. If you have multiple brothers-in-law, and they all have boats, you’re talking about your brothers-in-law’s boats. Don’t let yourself get intimidated just because you’re forming a “plural, possessive, hyphenated compound noun.” Just add “s” to the primary noun, and an apostrophe and “s” to the end of the word. Simple as that.
One final note: Even though you didn’t ask about it, the plural of “attorney general” is “attorneys general.” I think we can all agree it sounds weird … but there it is. Hopefully you found that edifying.