I completely understand how you feel because my AD abused benzos and Klonopin for a time. She has also thrown alcohol onto that mess too. It was a nightmare to see her stagger around and walk and talk when clearly she was not really conscious. Too many times we had to persuade her not to leave our house because we knew she could easily get hit by a car. Thank goodness she did not drive or atttempt to get behind the wheel of a car. If she had, I would have had to call the police. Eventually, she moved out because it was impossible to live with her.
On her own, my D gave up drinking, Klonopin, and benzos. I think she lives in fear of how badly they affect her. Maybe at some point your sister will decide to straighten out but you have no control over that. I would avoid taking a front row seat to her addiction if it troubles you.