You said that your SIL has a young daughter -- how young? If she is under 10, then she'd only pay the child's price.
The character meals are pricey (though not any more than many of the restaurants), and the food is usually good, but what you are paying for more than the food is the experience, the chance to meet the characters one on one, uninterrupted -- even if it's just for a few minutes at a time. So even if your niece wouldn't eat anything at all, the experience of taking her there and her getting to meet all of the princesses

-- especially Ariel, who is always hosting because it's HER place -- is priceless. She'll get pictures with each of the princesses, and autographs (make sure to bring a book!), and they'll chat with her one on one.
We took my DGD when she was three and I think she ate 2 grapes and a breadstick

Doesn't matter -- we had a lovely dinner, and she had an amazing time! We got adorable pics of her with Ariel, Cinderella, Jasmine, Snow White, and Belle. She was in heaven!
You did mention your niece's disability. I admit, I don't know a lot about the limitations of CP -- did you mean that she isn't
able to eat a lot of food on the menu? If so, you might talk to the restaurant to see if they have any sort of modified menu that would have food more appropriate for her -- like if she needs very soft food that is easy to chew, etc. But like I said, even if she didn't really eat anything there, if she loves the princesses, it's well worth going.
Oh, and if either of you has
AAA, they offer a small discount, I think 10%, for character meals -- you just have to let them know when you reserve and when you pay.
Also, the fairy meet and greet at DL is well worth the wait -- Tink and her friends are very sweet
