Notice the placement of the comma (after parents) in the following example:

Incorrect placement of comma and apostrophe

The correct punctuation is:

at her parents’,

However, sometimes the best solution is to rewrite the sentence (especially if you are uncertain about the correct rule).

After spending three months at her parents’ summer house, Jennifer was eager to return to the city.

Similarly, instead of writing:

Jennifer’s mother’s car was a pink Cadillac, and she loved driving it down Main Street.

consider rewriting the sentence without two consecutive possessive nouns (Jennifer’s mother’s) and the personal pronoun (she):

Jennifer’s mother drove a pink Cadillac, and Jennifer often borrowed it (with or without her mother’s permission) to cruise Main Street.

Don’t let incorrect punctuation muddy your writing. With a little effort, your brilliance can shine through!