Hi there, Andrea! Welcome back to the Disney Parks Moms Panel!
I can relate to your sister's concerns about finding vegetarian options on vacation. I was a vegetarian for many years, and I was always searching for creative meat-free choices. I can tell you from experience that I've never had any problem finding something tasty to eat at Walt Disney World.
Most counter service spots throughout the parks have some meat-free option. If your sister eats cheese, she can try one of my favorite choices, the caprese flatbread. You'll find variations of it at Pinocchio Village Haus in the Magic Kingdom and at the Electric Umbrella at Epcot. Another new favorite for me is the Lighthouse Sandwich at the Magic Kingdom's Columbia Harbour House, a tasty mix of hummus and broccoli slaw on multigrain bread.
As far as table service restaurants, I've found I can usually request a dish without the meat. My husband is a meat eater, so I'll often order the meat "on the side" and give it to him. Many have vegetarian menus by request, and often the chef will come out and meet with you if you're not finding anything you like. And of course, if you opt for a buffet such as Cape May Cafe at your hotel, Disney's Beach Club Resort, there are always lots of meatless options.
A great resource for dining at Disney is allears.net. They have menus of pretty much every morsel of food available on Disney property, with a whole section on vegetarian options.
I hope this helps your sister nosh to her heart's content, Andrea. Please visit again if you have more questions.
Have a magical day!
Kathy K.