My sister swung something at me. What should I do?

Well, you already told your parents so that's done. What did your parents say anyway? Whatever "dirt" she has on you is probably far less serious than her throwing a plate at you, potentially hurting you very severely. Hitting is one thing, but throwing a plate directly at your head is another. Her being the older one, she needs to know how to control her emotions. I think you did the right thing by telling your parents. What would you apologize for anyway? You didn't do anything wrong; she was the one who tried killing you! And don't worry about her exposing any kind of "dirt" on you to your parents...I'm sure she feels pretty bad about what she did. Wait for her to apologize and in the meantime, don't talk to her.