I have to agree with LaTeeDa.
The father MUST be told. She is putting her girls in grave danger. Yes, she will probably be mad at you, so .................. she has been before and will be again.
The children will be upset for a bit, but will come to understand that 'mommy is sick.'
You cannot 'help' your daughter, as much as you would like to, it just won't happen.
Instead, how about 'helping' yourself? Have you tried AlAnon, or some therapy with a counselor who specializes in addictions?
Please keep posting here and let us know how YOU are doing as we do care very much, and many of us have been where you are now.
Love and hugs,