Hello! I think it's wonderful that you're so concerned about your little sister, and aware that she could be in danger of an eating disorder. It's very important to catch an eating disorder as early as possible and you being so aware of her could end up saving her life!
I think the best thing for you to do is to make your parents aware of your fears. Ask them to help you keep an eye on your sister to make sure she's not heading into dangerous territory. Your parents are important people to have on your side because they're in the best position to watch your sister's behavior right now, and because they're going to be essential to helping her get well if she is slipping into an eating disorder.
It can't hurt to talk to your sister about your worries too. But be warned: if she is dealing with an eating disorder, she's likely to deny it. And being as young as she is, she might not take your fears very seriouslty. Involving your parents is probably to be the best way to keep her safe.