How do you stop a sister from being overweight if she wont stop eating or not listen to you?

Back off! She will deal with it when and if she is ready. Your pressuring her may do more harm than good. Each one is responsible for their own health. Tell her you love her and you will support her independently of her weight.
Overweight. This probably has become an emotional problem and you telling your sister to stop is making it worse. Instead talk privately with your mom and dad about how to get professional help for your sister. Good way to start is by calling your doctor.