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:
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