Sounds like a good plan, sorry to "break up" the family, but in the end everyone will be happier. She won't spend the rest of the trip looking sideways at you or rolling her eyes.

I am a sister too, so I can only imagine. You do get big points for researching this for her though.
I'm sure she'll enjoy California Grill. Its not clear if you are on the dining plan, but if you are not then it won't matter if she chooses a one or two "TS" meal. If she decides she wants a good vegetarian meal, with lots of choices I suggest she make a reservation for Boma at the Animal Kingdom Lodge. They cater to vegetarians and the kitchen is right behind the line so there is always someone around to describe the item so she can be certain of what she's eating. We tried it for the first time last trip and will be returning. Its one of the most "freeing" dining places for vegetarians at Disney. She can probably say yes to 30-50% of the buffet. Also the tables were fairly close together and found ourselves in conversation with neighbors on both sides so I doubt she'll feel like a loner there.
This is not to suggest that CG will be bad in the least, just another vegetarians' perspective should she decide to try something else.
As for the beans, especially on a cookout menu, they almost always have a pork or meat base. Just one of those things you know after 35yrs of vegetarianism!
You've crossed a hurdle and have a better perception of your sister's dining dilemmas, now just have a good time when you get there. Nice chatting with you.