She started laughing hysterically, and I had to shove her out of my room. Now she won't get off my back! I can't get any masturbation in because every single hour she is walking into my room. She also makes jokes with her friends and even at the dinner table! I am so embarrassed and don't know what to do. How can I get her to stop, without making myself even more embarrassed?
Sounds like a bad situation! If your parents have already been subjected to your sister's blabbing (and it sounds like they have), as embarrassing as it may be, perhaps it's time to ask them to intervene. Your sister needs to understand that a person needs privacy, and your parents are the best people to convey this to her. You don't have to tell them you masturbate – just say your sister has been invading your privacy and you want it to stop. (And ask for a lock for your door!)
If you don't want your parents involved at all, have a serious, sit-down talk with your sister and discuss the idea of privacy and explain how important it is to you. If that doesn't work, I would rather not suggest this, but your sister may need some of her own medicine. I'm sure you can think of subtle, non-destructive ways to invade her privacy and show her how you're feeling without overdoing it. Just try not to let matters get out of hand; remember, you're trying to solve problems, not make matters worse.