Brothers-in-Law

If I have more than one brother-in-law, I have brothers-in-law. There is more than one brother, not more than one law.

Similar examples include sergeants-major, Surgeons General, and congressmen-at-large.

To create a possessive form, add the apostrophe-s to the end of the compound noun.

My brother-in-law's house. The Surgeon General's warning.