PLEASE HELP MY SISTER

First of all, I really don't think the word you're looking for is diabetic.... Tell your mom and dad WHY she should eat. All the health impacts of potential ANOREXIA. Thinning hair, energy loss, etc., I don't know how you'd talk to a doctor without your parents there, but if you see one, tell them about it and see what they say. Get them to tell your parents what they say. I wouldn't tell your sister the health problems, she'd be too young to understand it. It'd just scare her. But try to persuade her to eat anyways. Tell her how good food tastes, ask her why she won't, and the likes. If she doesn't eat for too long, by all means, YOU force her to eat!!! Kinda cruel, but it's way better than nothing. Hopefully she'll start eating soon again.

I don't think diabetic is the right word.
Does she drink? You can get nutrients in her that way.
Are you sure she's not eating anywhere else? She could be getting food elsewhere which is why she wouldn't eat at home.
Is anywhere in her mouth or in her digestive system hurting? She could be refraining from eating to avoid pain.
If the answer is no to at least the first two, there's a huge problem. You either have to find the main reason why she isn't eating or force her to eat. Eventually, she'll end up in the hospital dehydrated and not much energy (assuming your parents actually take her there). By then, doctors probably would feed her through tubes or something.
If its possible, when she's sleeping, you can try to sneak in some drinks in her. A person can survive up to a month without food, and about a week without water. ..Um, not sure about kids though.
You might want to speak with a teacher, school counselor, or any adult. You would want any help as soon as possible. I've never been in a situation like this, so this is all I can really say about it.

How long has it been since she ate? And are you sure she isn't eating something when you aren't there? Don't worry, she won't become diabetic - diabetes (both kinds) is causes by a problem with the insulin in your body, either there not being any or there being something wrong with it.

Most children get TYPE 1 diabetes at that age, which has nothing to do with eating sugar or being unhealthy. (It's just something that unfortunately comes up, I got it at 8 years old), anyway, it's best to take her to the doctor, if your mother doesn't do anything about it the simplest thing to do is to go doctors and he/she will either tell you what the problem might be, or tell you how to make feeding her a bit easier.

hide everything sugary away from her sight
don't worry if she gets hungry she will eat eventually