Hi Julie,
Welcome to the Disney Parks Moms Panel and thank you for asking us a question!
If you’d like to add a
Disney Dining Plan to your upcoming visit, it needs to be purchased for every person staying in your Walt Disney World Resort hotel room. I’m not sure how many hotel rooms your sister-in-law reserved for your entire party but if your six family members occupy one or two rooms independently of the others, you may be able to add a Disney Dining Plan to your reservation, unless it was booked using a
Special Offer and can no longer be modified.
I would recommend contacting Walt Disney World Resort in order to see what your options are, including your payment options, as I don’t believe that you’ll be able to make this change online, even if your Resort reservation is linked to your My Disney Experience account.
I hope my answer helps you plan your upcoming vacation. Please don’t hesitate to come back and see us if you have any other questions.
Have a Magical Day,
Lynne