Being an alcoholic/pill addict is an awful way to live as you have witnessed.
Your sis is once again struggling to overcome. Try and show her all the compassion
you can. She may have tried recovery before and failed, but NOW she is trying again.
My son died from the disease of addiction/alcoholism this summer. I don't regret one moment that I supported him or visited him the three times he went to rehab to try to beat this disease.
If your sis was inpatient with any other disease you prob. wouldn't feel as conflicted in showing support. Addiction is potentially just as fatal. I hope your sister gets some long-term sobriety this time. Sometimes all we have to offer may be compassionate words and thoughts which go along way to help them in the struggle.
Your sister is fortunate to have your love.