DaisyL, you say that your stepdaughter says that if she goes into rehab, she will drink again as soon as she leaves. However, if you could get your stepdaughter into rehab, it may get her to re-evaluate her actions and start her on the road to recovery. It is difficult to get someone into rehab though and incredibly expensive if you have to pay for it.
I'm so sorry for DaisyL, paying the emotional price for caring about an addict, for her stepdaughter and especially her stepdaughter's children. I think it demonstrates that the power of addiction is greater than parents' love for their childen. I can't imagine that the glasses of wine that I used to enjoy could have the ability to stop me caring about my child. It must be a terrible force indeed.
Then, even if the addict beats their addiction, they have a battle to live a normal life in a society where alcohol is everywhere and where they will always be condemned for their 'weakness' and/or 'choice' to become addicted to a substance that most of us commonly enjoy.