My father died also with circumstances much like yours, however, I believe the drink, his brandy/vodka/bourbon whatever he could get his hands on lead to him to forgetting to take his heart medication. He was my SUN when I was a little girl. My EVERYTHING. As his disease progressed his behaviour became HORRIBLE. I stopped a police car once when I fled the house at 16, begging them to DO SOMETHING but they could do nothing, he had not hit me or threatened me. My mother had him hauled off to jail once, with help of the police. They said, get him off the property and we can arrest him for public intoxication. She did and they did, right in front of the house. Terrible times. The sadness comes to me from the loss of the relationship i once had with my father 40 years ago, when I was a kid. He died 20 years ago and although I mourn the loss of my Dad. I do not mourn the loss of the alcoholic who I knew best the last half of his life. I understand how you feel. Everyone grieves in their own way. Some grieve because some things can NEVER be undone, all chances are gone. Some invite grief to move in and are lost without it.
Let your sister mourn the way she needs to, you certianly dont need to be a part of it.