What should I do? I'm afraid my sister will commit suicide.?!

My sister is 30 years old. When she was 5 and I was 14 our mother committed suicide, without warning. She was raised by my father in a normal environment. For whatever reason (I believe overcompensation) she turned out to be trouble. She is a drug addict and does a lot of other things probably associated with the addiction. She is an RN and has two children but was recently arrested for drugs and numerous theft charges. She was out on probation on some of the charges and failed her drug test and was put in jail for six months. She had about 15 other charges pending and has been convicted on those. She has sentencing 2/25/11 and the guideline is 3 to 7 yrs. She is out pending sentencing and living with a drug addict. I suspect she is using again. She has mentioned killing herself to our other sister several times. I called her and talked to her about it and explained that if nothing else she should think of what that would do to us. I said things like you know what you have to do here but you don't know what the other side holds, etc. Yesterday my sister received a letter that was simply typewritten lyrics to two songs about dying and a letter about "drug of choice". It takes the form of the drug writing to her and telling her how bad she is. It ends with "it's not over until you're dead". There was no explanation. I'm so scared she will do something, but I don't know what to do. She lives hours away from us and she's not answering my calls. She says I am mean to her. I do tend to not buy into her "crap" but I'm not mean. I just try to apply "tough love". She is really a person you can't reason with. Can someone offer advice?