It's very difficult standing by and watching a loved one destroy themselves. I have been standing by and watching my mother destroy her self over and over agian with her addiction, I have even corrupted myself with drug use and abuse. What I've learned though through these years with my Mom is to not judge harshly but to try to understand her illness and be supportive, to encourage her to make better choices and to praise when she does. My Mom will never live a sober life, I accept this now, but there are periods when she does very well and during these periods I constantly support her to take one day at a time and to continue approving. Try not to be harsh, just tell her your on the outside looking in and that you care about her well being and all you want is for her to have a good life. I hope things get better for both you and your sister.