Don't Upset the In-Laws
Remaining in the apostrophe family, one of the pesky little questions in the English Language is how to handle In-Law forms. Keep the family happy; write and pronounce those very important titles properly! :)
It's confusing to know how to correctly differentiate between plural and singular possessive forms of compound nouns. And when your spouse's family are coming to visit, remembering how to write or say the in-law form with all its dashes can be annoying.
The correct plural form for most compound nouns is to add an "s" to the main word. So, when you have "Mother-in-law" you want to add an "s" to the word mother. So, it's: Mothers-in-law or Fathers-in-law. (Not mother-in-laws or father-in-laws as a plural form.)
The trick to remember: More than one mother? You place the "s" on the end of the word, or being, that is plural.
A sentence: The fathers-in-law of the two friends did business together in the past.
However...if we're talking possession, then the apostrophe "s" goes at the end of the last word, in the following case, law.
My brother-in-law's boat got scraped on the hull by an oyster bed.
I use this form quite often as for one of my contracts, I have not one, but two Editors-in-chief of a publication I copyedit and help run: one editor-in-chief for a different area of law covered in the magazine. So, they are formally titled Co-Editors-In-Chief. The "s" is at the end of editor when I refer to the two of them; however, when I refer to the titles in a possessive form, I place the apostrophe "s" at the end of "Chief."
A sentence: I received my Co-Editor-In-Chief's reviews of the Spring issue of our magazine, today.