Lots of good suggestions so far. A couple others that may help:
- Check the kid's non-fiction section of the library for books on healthy vegetarian eating. There are enough vegetarian kids that our library has a whole shelf of these targeted to teens.
- Get her interested in cooking and meal planning. Again, the kid's section of the library is your friend. My 9yo is also really into the Rachel Ray kids cooking show. She even wants to go on the show. So DH and I have told her if she wants to put in an application, she needs to learn to like more foods and learn more cooking techniques. My kids get to watch as much cooking channel as they want.
- Frame things positively. Try not to say, you shouldn't eat X. Tell her, you can have all you want of A, B, C and D. Put a list of "Anytime" foods on the fridge/pantry to help jog her memory. We're doing this with my 7yo who has a suspected dairy intolerance. Instead of telling her what she can't eat, we labelled all the foods she can eat in the fridge and pantry with green painter's tape. Makes it super easy to pick out the good stuff. Over time the dairy foods that she likes will just disappear from our house.
- Don't worry about restricting your other daughter. This is a family issue and the family's diet needs to change. Yes, it will suck that she can't eat much sugar cereal anymore. She will live.