KAtie,
I am sure that this post has died by now, but here is my two cents worth;
As a high school coach ( girls) for the past 20 years, one way to help keep your sister away from all those bad things is to help her find altenatives. Like you mentioned, I believe, with her work at the hospital. Many times kids find drugs, drinking, sex, because they do not have another focus in life, and they really are just bored.
This doesn't always work, but I have found that giving alternatives, and it sounds like ( in your own life, at least ) that you have had alot of varied experiences, can at least give kids an option away from all the bads things she is into now.
Good luck.