Hi,
My understanding is that possesive nouns always get an apostrophe. It wouldn't look right to use an apostrophe with a possessive pronoun, but is the same true, do possesive pronouns ever get an apostrophe, or never?
If you are asking whether you can ever say things like
This sweater is her's
or This sweater is hers's
the answer is that you can't. Can you think of an example where you might?
Best wishes, Clive