Hi Shannon, welcome to planDisney!
You are visiting Walt Disney World Resort with your sister and niece, which tells me magical things are about to happen! I recommend spending some girl time with Cinderella at
Princess Fairytale Hall in Magic Kingdom Park. Step into the storybook at
Enchanted Tales with Belle and wave to Mickey during the
Disney Festival of Fantasy Parade. Matching shirts are not required, but are highly recommended!
Thank you for explaining your situation so clearly. I understand you want to add your niece to your Family & Friends list and that your sister manages her My Disney Experience account profile. It's important to connect with your entire party as Family & Friends so you can view and make plans for each other throughout your vacation. Congrats on taking the important first step of adding your sister to your Family & Friends list.
When you click "Add a Guest" at the bottom of your
Family & Friends list, Shannon, please confirm that your niece's name does not appear when "Find through my connected Guests" is selected. Assuming that's the case, your sister needs to adjust the privacy settings of her Family & Friends list, which will allow you to connect with your niece. Ask your sister to click "Update" next to her name on her Family & Friends list. She should then click the blue, "Share My List" button, which will make the words, "My list is shared," appear next to her name.
Once your sister has shared her Family & Friends list, your niece's name should automagically appear for selection when you once again try adding a Guest through your connected Guests. In my experience, a webpage refresh may be needed. If your Stitchuation persists, a helpful Cast Member at the
My Disney Experience, MagicBand, and MagicBand+ number would be happy to assist.
Enjoy your girl time, Shannon! Girls just wanna have magic *:･ﾟ✧*:･ﾟ✧
Plan magic; planDisney.
Jennifer