If your sister doesnt listen to you in general, then your efforts may not be helpful. Talk to some of her friend whom she listens. If you will make her understand your point, she will get negative towards you. And this will worsen the problem. So better let some other people talk to her. In cases like these, people generally behave impulsively. Before she gathers more negative experiences, its better she learns what she is doing wrong. I believe its her personality which is compelling her to take risky steps. In such cases, counseling will help. But she needs insight.