Oh there are MANY good places to eat for a vegetarian! In MK my favorites are crystal palace (buffet) and columbia harbor house. They have a sandwich made with hummus and coleslaw that is very good. I also hear their vegetarian chili is good but I haven't tried it yet. Maybe in 4 days when i go back to the world!
In MGM there is mama melrose which my boyfriend and I like very much. They have an eggplant dish that is really good. We're big eggplant fans. There is also Prime time cafe which has a vegetarian stuffed pepper that is OK. I would go first for the Mama Melrose. At Sci Fi dine in they have a vegetable dish that I thoroughly enjoyed, and they have a spinach artichoke dip that is also really good.
There's a number of places to eat at epcot. My boyfriends favorite is San Angel Inn. They have a separate vegetarian menu which has two meals on it. The plato vegetarian which we had was really good. We were very pleased. Sunshine seasons has a few different options, the vegetarian sandwich is very good in my opinion. My boyfriend would not eat it though because it has mustard on it. He's not a fan of mustard. Also alfredos (though i don't know about the *new* alfredos) food was good, we had (what else!) fettucini alfredo. They don't call that place alfredos for nothing!
My favorite is boma at the Animal Kingdom Lodge. It's a buffet that offers a lot of vegetarian options. They have some decent hummus and breads, they always have atleast one vegetarian soup (butternut squash is my favorite), they have felafel that is good, potatoes, lots of stuff.
If you check out the dining review boards right now there is an excellent post about 3 vegetarians who dined for 8 days, the reviews are really good, with pictures, so maybe you could show your sister those and she can decide if she wants to eat at any of those places. Also check out the allears menus where you can read descriptions of each meal and figure out what she might like. There are a TON of vegetarian posts on these boards so if you do a search you are likely to turn up with a lot of stuff. Good luck
tricia.