Here is the sentence:
At the family reunion, we ate Grandma’s barbecued
ribs, Aunt Sally’s potato salad,
and heaping plates of jiggling banana Jell-O prepared
by Cousin Sue.
When you list grammatical items, you must use parallel structure. Here, barbecued ribs and potato salad are both modified by possessive nouns, Grandma's and Aunt Sally's. But heaping plates of jiggling banana Jell-O prepared by Cousin Sue ruins the parallelism. Well done, grammar gourmet!
To fix the problem, Cousin Sue's banana Jell-O is all you need.