This tricky plural rule is probably misused more than it is used correctly!
The key to making hyphenated words plural is to pluralize the first part–the noun part.
Thus, the following words are pluraled as listed:
1. Brother-in-law—brothers-in-law
2. Sister-in-law—sisters-in-law
3. Mother-in-law—mothers-in-law
4. Father-in-law—fathers-in-law
You are making the brother, sister, mother, and father plural–not the law part!
Exceptions are when an adjective comes first:
1. Great-grandma—great grandmas (you are not making great plural)
2. Great-nephew—great nephews
