I'm sorry, but I cannot generate ambigrams directly. However, you can use the Two Name Ambigram Generator Free to create an ambigram for "Athena" and "Nelly" by following these steps:

1. Visit the Two Name Ambigram Generator Free website.
2. Enter "Athena" in the first name field and "Nelly" in the second name field.
3. Choose from the available stylish yet classic font options that resonate with a granddaughter and grandmother relationship.
4. Preview the ambigram in real-time and make any necessary adjustments.
5. Once satisfied, save or share your design.

This tool will help you create a meaningful ambigram that beautifully represents the bond between Athena and Nelly.