Melissa,
A big pat on the back to you! You made the right decision for sweet Presley! You can have a 6 month party for her without her sister. I also agree that having friends like that is probably not worth having.
When I was 17 years of age, my best friend started doing heavy drugs, drinking and dabbling in witchcraft. I told her I would always care for her and if she ever needed someone to help her, to give me a call. At the age of 39, she overdosed and died. I never had contact with her again after I broke off our friendship although people around town would often speak to me about her. She was very mad at me but I did not need involved in that kind of life style and knew she would drag me down with her if I stayed friends.
I went to her funeral and was shocked at how pale and old she looked. Walking away was a very difficult thing to do but probably one of the best decisions, people wise, I have ever made in my life!