It does not sound like she is eating enough.
I eat raw veggies, fruits, soups, and some starches when I want to loose weight. I still manage to eat A LOT of it then (like two heads of lettuce a day, 8 bananas a day, etc.)
The thing is that she is eating foods that have low density of calories per volume. What makes a person *feel* full is the volume of the food. However, what makes people *stay* full and energized is calories. Thus, if you want to feel full and loose weight, eat foods that have lots of volume, but low calories. That is a diet like your sister has right now. If, however, you intend to keep on weight, or add weight, or are a growing teen, or a child, or have a small stomach, then eating foods that are really DENSE in calories is the key.
That means adding lots of fats, or eating foods that are more processed, or heavy sauces, or more cooked foods (where the water is cooked away), and/or lots of nut butters (more dense then raw nuts, which help loose weight with all that chewing).
I would usually stay away from those types of foods, but in your sisters case, it might be good to go there just to keep up caloric needs met.
Something that might work for her: try vegan alfredo pasta. Or try deep fried battered eggplant. Or fried boca burger on a toasted bun layered with vegan cream cheese, tomatoes, and lettuce topped with onion rings and margerine battered mushrooms. Or a deep fried bean and rice burrito with lots of avocado on the inside. Go eat chinese and get the deep fried tofu with their really heavy veggie sauces. Get some scallion pancakes while you are there.
Do not cut down on sugars or fats. When I first went vegan, I used to polish off a whole lb of pasta by myself until I figured out how to thrive on veggies and legumes.
This is not a recommendation that works for everyone. But she does not need to starve. Her diet actually sounds so fantastic for long term health. But it will not work until her body adjusts to less dense calories. That means stretching out her stomach so that she eats more volume a day to make sure she maintains her calories.
I would avoid making a slow transition. Just go for it!
"The worker has the right to leave his boss, but can she do it? And if she does quit him, is it in order to lead a free life; where she will have no master but herself? No, she leaves to sell herself to another employer. She's driven by the same hunger. Thus the worker's liberty is only a theoretical freedom, lacking any means of realization; an utter falsehood."