Hey there,
You sound like a great sister to have, to care so much about her. I can't tell you if she has an eating disorder or not, but her eating habits do not sound healthy at all.
You might want to try talking to her, let her know you are there for her, and that you care about her. I'm not sure about telling your parents, but if she gets much worse, you might have to. Not eating things with sugar in it, isn't always a bad thing. Eating healthy is okay, but if she seems afraid of sugar, then that is not alright.
I have anorexia, and I have an older sister. So I'm going to try and explain this from a different angle, from that of someone who has an eating disorder. Whenever someone would approach me about my eating, or tell me to eat more, I would get angry. It's not that I did not always want to eat, but I could not eat. Eating disorders about about control, mainly. Control over your body, your life, your mind. Sometimes it gets to the point where you can't see anything else, all you see is fat and imperfection. You need that control so badly, you will do anything to get it, even if it kills you. Often, eating disorders have underlying problems, something that someone might be trying to block out or make go away. I was angry when my sister told my parents, very angry. But it did help me, and it still does. Knowing that people care enough to piss you off because their worried, it means everything.
You are a good sister, and I know you are worried. The best thing you can do is be there for her, catch her when she falls, and keep telling her that she's not alone, because I'm willing to bet she feels really alone right now.
If you need anything, feel free to
pm me.