Wow. that's a tough situation. i am the middle of 5 children and my older sister was diagnosed with special dietary needs when we were children. my mother made the switch for all of us, no more real chocolate chips, she substituted carob. we didn't have regular desserts, we had things made with various nectars and such. TVP was regularly added to many dishes, we stopped eating some foods, added new ones, etc..... i have to tell you that at first i hated it and i hated that my sister, one person, was the reason our entire family of 7 had to eat things with so little taste.... fortunately these days there are more options for people with special dietary needs (and they taste much better as well). i do agree with the other responses that your approach to the situation is vital. my mom never made it just about my sister. she made it about our family and the need to eat in a healthful way that honored all of our bodies and their needs... we children still made the connection though and dearly missed our old favorites, but we've all made it safely to adulthood with no food related casualties.
my mother made it an adventure for us and that i believe lessened the blow of not being able to have some things we used to enjoy. this was not just for my sister's sake, but i know it really helped her. we were encouraged to try new things and my parents kept it fun. they would talk about where the food came from, who else (what part of the world) would eat it, etc. so i had fun imagining another little girl who enjoyed quinoa and things like that....i know now that it must have been a real challenge for them at the time, but i never would have known that as a child. i hope this helps you!