What is the plural possessive of brother-in-law?

brothers-in-law's

The plural possessive form of "brother-in-law" is "brothers-in-law's".

To form the plural possessive of a compound noun like "brother-in-law," you first need to make the noun plural by adding an "s" to the end of the word, resulting in "brothers-in-law." Then, add an apostrophe followed by another "s" to indicate possession, resulting in "brothers-in-law's." This tells us that something belongs to multiple brothers-in-law.