Below are rules to show you how to use possessive pronouns.
Apostrophes are not added to possessive pronouns because they are already in their possessive form. 
Incorrect: her's or hers'
correct: hers
Possessive pronouns are used to show who or what owns something. 
Remember that a possessive pronoun can replace a possessive adjective and a noun to avoid repeating information that has already been made clear.
Theirs can replace their car. 
Hers can replace her brush. 
A possessive adjective appears in front of a noun as in: 
                                That is their car.   (possessive adjective their + noun car) 
A possesive pronoun replaces a possessive adjective and a noun.
                                That is theirs.     (possessive pronoun)
Language Arts:  Possessive Pronouns