I suggest you say something like this to your sister:
"Hey, [sister's name], I need to talk to you about something. I've noticed that you've been chewing on my pencil sharpener, and I'm really worried about it. It's not safe to eat something like that, and it's also not hygienic. Can you please stop doing that? If you need something to chew on, I can give you some gum or a piece of candy instead. But please, don't eat my pencil sharpener anymore."