Thanks for posting this, Steve. We all have demons, just some have worse ones than others.
I have a family member that is going down this same path. Unfortunately, everyone around her is enabling her, even though all of them know what it does. I only hope that she sees the light before it is too late.
You can't do anything for a person like this. They must do it themselves. The best you can do is to be there for them when they quit.
My prayers for your sister.
Try and don't regret (although you are right, ultimately they must do it themselves).
Steve