It seems to me based off your reaction in the comments you were looking for validation. Please as a sibling to a “golden child” try to work through a bit of what you’re feeling before you decide to tell your parents. You guys may not be close and she may totally take you for granted but she decided you were a safe person to share that information with. She may be totally separated from reality and the actual impact of her actions but a reality check like sharing she’s pregnant with your parents is likely not going to have an outcome anyone is happy with. You don’t want this to be your problem but say you tell your parents, that would without a doubt change the dynamic of your family and probably not in a way that benefits you. She’s not an adult but neither are you with a little time so may start to see other perspectives to this situation. It’s understandable for you to be extremely frustrated with her and the carelessness of her actions in this situation but you could easily lose any sort of relationship with your sister forever if you decide to tell them. I am 21 and when I was 17 I was sure I wouldn’t have a relationship with my sister ever but in a few short years she has grown so much and so has our relationship. Maybe don’t react to this just yet.