Rule 7
If the compound noun is plural, form the plural first and then use the apostrophe.

Example:
my two brothers-in-law's hats

Rule 9
Never use an apostrophe with possessive pronouns: his, hers, its, theirs, ours, yours, whose. They already show possession so they do not require an apostrophe.

Correct:
This book is hers, not yours.

Incorrect:
Sincerely your's.

Rule 10
The only time an apostrophe is used for it's is when it is a contraction for it is or it has.

Examples:
It's a nice day.
It's your right to refuse the invitation.
It's been great getting to know you.

Apostrophe rules part 3

Rule 8
Use the apostrophe and s after the second name only if two people possess the same item.

Examples:
Cesar and Maribel's home is constructed of redwood.
Cesar's and Maribel's job contracts will be renewed next year.
Indicates separate ownership.
Cesar and Maribel's job contracts will be renewed next year.
Indicates joint ownership of more than one contract.