Find out that it is that your sister wants from you in order to shut up. After she tells you what it is, if it is worth it for her to shut up, give it to her. However, in order to preserve yourself from blackmail, it would be better if you burst her bubble by confessing that which she is trying to blackmail you with. If she is the kind that asks many questions, just refer her to someone else.
Your sister is looking for attention. Try speaking with your mother. CALMLY tell her how you feel that she is really irritating you. Her behavior now could your relationship with her in the future. Your mother needs to explain to her that her behavior is unacceptable and it won't be tolerated. Be prepared...most parents don't do anything about this. My mother would not listen. I made it clear to my kids under no circumstances would this be tolerated.