I really feel for you, Daisy, as I have been similarly placed with my siblings.
I hope things turn out all right, but believe me, the decision you have come to, is the right one.
Some people really need to touch rock bottom before they are able to change, and I think a very great number of addicts, irrespective of the addiction, find it difficult to listen to family. My sister stopped drinking because a total stranger, a nursing sister, God bless her! told her precisely what the whole family had been saying for years.
From us it had had no effect, from a nurse on night duty, it did. Something similar might just be going on with your step-daughter.
An alcoholic who has taken the decision to stop drinking, can, but will need support, one who feels pushed into it by all the rest of us well-intentioned people, can't stop.