How to get a little sister to understand shes younger than you?

Just tell her that you are an adult and she is underage. Let her know that once she's of legal age, Mom and Dad will loosen up a bit. It could be that your sister may have looked up to you most of her life and as been seeing changes in you as well as changes in her. Be her friend, this is a phase she'll probably grow out of once she starts college.