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 你 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 食物 tastes, ask her why she won't, and the likes. If she doesn't eat for too long, 由 all means, 你 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? 你 can get nutrients in her that way.
Are 你 sure she's not eating anywhere else? She could be getting 食物 elsewhere which is why she wouldn't eat at home.
Is anywhere in her mouth 或者 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. 你 either have to find the main reason why she isn't eating 或者 force her to eat. Eventually, she'll end up in the hospital dehydrated and not much energy (assuming your parents actually take her there). 由 then, doctors probably would feed her through tubes 或者 something.
If its possible, when she's sleeping, 你 can try to sneak in some drinks in her. A person can survive up to a 月 without food, and about a week without water. ..Um, not sure about kids though.
你 might want to speak with a teacher, school counselor, 或者 any adult. 你 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 你 sure she isn't eating something when 你 aren't there? Don't worry, she won't become diabetic - diabetes (both kinds) is causes 由 a problem with the insulin in your body, either there not being any 或者 there being something wrong with it.

Most children get TYPE 1 diabetes at that age, which has nothing to do with eating sugar 或者 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 你 what the problem might be, 或者 tell 你 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