Oh, I wish I could remember the name of the book or author... there is a book that would be perfect for your sister.
The premise is that you divide the responsibility of food as such:
Parents - decide what food to make, what goes on the table. They are not short-order cooks (though of course they will consider their chidren when meal-planning - but not CATER to them).
Children - decide what food on the table they will actually eat, and how much.
This removes a lot of control battles. For the parent's part, they need to stop cajoling and nagging and bribing and threatening. Just sit down to dinner. It works best if the food is all in serving dishes on the table for everyone to see and perhaps be tempted by. I tend to try to save dishes and just serve from the stovetop, but I have personally seen my 4 year old take some interest in food when it's displayed on the table. Nothing miraculous, but she might ask to sniff a dish or lick one bite, and maybe even take a bite or two. Over time she might get used to it and actually fully eat it.
Anyway, the parents don't try to force a kid to eat anything. The kid is in charge of her own mouth and can fill it as she pleases, based on the choices the parent has provided.
According to the anecdotes, at first they usually go hungry. The children are used to the power struggle. Assuming no very serious health problems (like serious metabolic disorders or something), this is fine. The child will get hungry and start to relax if the parent stops trying to force feed etc.
The child decides how much food to eat, and which specific food that is offered to eat. For example, the child might just eat the brown rice but refuse the green beans. Fine. No need to make a fuss. The child will not like green beans because they are forced to eat them. They may, however, experiment down the road. Also the parent is free to "sneak" certain foods into liked foods (like blending vegetables into spaghetti sauce) if they want, but they can't force, cajole, bribe, etc. If the kid eats the spaghetti sauce and they don't know there's spinach in it, fine
Parent also decides "when" to eat. Of course the parent must plan appropriately for the child, and set a reliable schedule and include snacks at reasonable points. But the point is that the child cannot demand a snack 10 minutes before dinner is served. They'll have to wait for dinner. They should be able to rely upon consistent meal and snack times. Their hunger will adjust.
Also, it's pretty critical that the kid eats meals with a parent, if not the whole family, and at the table and not in front of the TV. This increases their awareness of and interest in food. Also it increases the amount of time with the focus on the food - in front of the TV is pretty much about shovelling it in, and keeping your attention on the show (hey, you're hearing this from me because we've done it). At the family dinner table, the pace is slower, you talk, you digest. My 4 year old is more interested in food she sees us eat. Proximity to the food and its smells seems generally to help her get used to it too, she finds it less offensive.
We're using this method and while I can't say it's wrought utter miracles - my kid is still picky - it IS better, less stressful, and she's eating a wider variety of foods. When we catered to her she would eat crap exclusively - we'd ask her what she wanted and she'd pick cereal or frozen pizza every time. But now that she's not consulted (we only consulted her because she's so picky and didn't eat anything!!!) she eats a wider variety of things. She still doesn't eat aspargus or anything, but you know what? She'll eat the pasta with the asparagus picked out. Meaning, she can cope with being given a dish with some ingredients she doesn't like, and just work around them (if they are large enough to work around, that is).
Hopefully someone can come along and give the name of the book, because I got the recommendation on MDC myself.