In an earlier blog post we looked at compounds and and noted that some are written with one or two hyphens. Here are a few examples:

Brother-in-law (your sister’s husband or your wife’s or husband’s brother)
Runner-up (one who finishes in second place)
Cul-de-sac (a street that is closed at one end)
Editor-in-chief (the manager of an editorial staff)

How should you write the plural form of such compounds? The answer is fairly logical: add the plural -s to the main part of the compound, the significant part.

Brothers-in-law
Runners-up
Editors-in-chief

Cul-de-sac has two plural forms: culs-de-sac or cul-de-sacs

When we write the genitive form, the -s comes at the end when we talk about people: 

My brother-in-law’s new car
The editor-in-chief’s wife

However, you can also write

The wife of the editor-in-chief

When we talk about things, we use the genitive form with of:

The end of the cul-de-sac