The big sister?

If she is fighting alot, then she must be feeling alot of anger and frustration. It's good that you are talking to her alot. But try to isolate the problems that she is facing. Work with her as a sister, a friend and a mentor to conquer each problem. Tell her the consequences of her actions both short term and long term. Try not to sound judgmental or mad at her. What you're doing already is great and she's lucky to have a sister like you to care for her. Keep it up. Be patient and things will turn around.