Very sorry for your loss.
I did watch my mom deteriorate from ARDS, acute respiratory distress syndrome. She took a prescription drug overdose, it was seroquel. She was taken to the hospital where they pumped her stomach. She survived that part of it. But when she vomited, she got some of it in her lungs. This is ultimately what killed her. Well, she would have lived on the respirator, but they thought two months in this condition was long enough. They had to sedate her to unconsciousness or a coma like state while she was in ICU. So, kind of a vegetative existence really. I'm not sure if she was aware of much. That part haunts me, wondering if she knew what was going on when they took her off life support.
Anyway, she was an alcoholic and battled it her whole life. I just always thought I'd get a phone call that she died in an accident. I never dreamed I'd have to watch her on a hospital bed in that kind of state, and then be the one to have to make the decision to take her off life support. It's quite a burden to bear.
I am sorry you had to see your sister this way. I can empathize with you.