Question

What is the correct singular possessive form of brother-in-law? What is the correct plural possessive form?

Answer 1

Singular possessive: brother-in-law's

Plural possessive: brothers-in-law's

Explanation:

The plural of brother-in-law is brothers-in-law because the base noun is made plural.

However, when forming possessives, the compound noun is considered as a unit.

Hence the singular possessive brother-in-law's and the (awkward) plural possessive brothers-in-law's. The latter, as stated, sounds awkward and could be replaced by a restructured sentence to convey the same meaning.