Katie,
One thing I would suggest is to be a bit more selective about how much you let your sister in. I can understand wanting to have a best friend type of sister relationship, but perhaps for a time it might benefit you to keep your private life less accessible to her. In other words, she can’t “tattle” if she doesn’t know what’s going on in your life. I’m not suggesting you shut her out completely, but let her earn your trust. I’ve had to establish boundaries with both family members and coworkers in terms of sharing my life. Unfortunately, there are people who will exploit your openness for their own selfish reasons (whether intentionally or not), so it might help to keep your cards closer to your chest until she matures and proves herself to be someone you can trust with private matters (such as your sex life). If the things she’s saying are completely baseless and fabricated on her end, well, there’s nothing you can really do to stop her from telling tall tales. Rise above and try hard not to engage. It seems she’s looking for a reaction from you and will press your buttons until she gets one. This is a endurance test in patience, that’s for sure.

Good luck!