What would you do when put in this position....?

Unfortunately you are just a minor, but i do understand where your Anger is coming from. Your sister is 19, shes an adult, she has her 5 senses put right, she knows what is good and bad for her. Now, If your sister does not want to be his girlfriend anymore, thats up to her. She has to make a decision herself, you cant decide for her. Maybe if you try talking to her about this she MIGHT listen to you, or she might not. do not jump into conclusions yet, take things step by step and let her decide for herself. Good Luck to you and your sister! :)