I need help!! its about my sister! Please!?

I have a young sister, Sarah and she has not been talking to me for a year now. we live separately. she lives with our parents still. I have my own place. She does not want to contact me in any way now and I don't know how I upset her. I was told from someone in the family that she has this new boyfriend and I think he is what is changing her. She has become absolutely devilish and my friends have seen her dating lots of diffrent guys recently(not her bf) and now she barely wears anything when she goes out and with the tons of makeup she wears, she looks like a slut. She was supposed to go to university but now she says she doesn't care about studies and just wants to get away from home. Sarah puts al those almost naked pictures of her on fbook! I am ashamed of what she has become and I don't know what to do. I cry everynight because I worry for her and our parents have no control over her because she threatens to kill herself if they do not let her do what she wants and she has even cut herself once. Also Sarah said all those hurtful things about me to my parents and I don't know what to do... I have been searching the internet for similar cases but I think I might be the only one with a slut for a sister. she does not want to see a psychiatrist.