Well, I haven't had exactly the same situation, but my mom had my Dad leave when I was 12; she then married a guy who was sober and steady--a great stepfather. She divorced him actually when I was in my early 20s because he fell off the wagon. So, our situations are not quite the same, but as a two-time Dad-loser, I can tell you what they might feel when you leave:
1) If you do have a great relationship with them, they will feel your loss. Anything you can do to minimize that, like providing them with visits, phone calls, etc., to minimize the abandonment feelings would be very helpful. If you can get them family counseling, that might help.
2) You must be very honest and clear, and at the same time staying away from blaming their mother. You're walking a fine line, because if you minimize the truth about the drinking, they will wonder why you're leaving, but if you tell them the truth, they may be frightened and confused--especially if they don't realize that she's alcoholic.
And now that I'm actually writing this out, maybe in addition to a lawyer, you might want to get the advice from a family therapist about how best to handle this. It's easier, I think, to explain to a child what my mother explained to me: "Daddy's sick, so we're not going to be married anymore. His drinking is causing problems in the family so he won't be living with us." That made sense. I think if she had said, "Daddy's sick, so we're not going to be married anymore. His drinking is causing problems in the family, so I'm leaving, and I'm not taking you with me" -- that's a little traumatic for young girls to hear. You will do them a great service if you do whatever you can to detach from their mother, but provide them with whatever continuity of your relationship with them that you think is possible and appropriate. I'm going to be honest--it's hard on children to lose stable parent-figures at the ages they're at now. Any age is hard, but I have a my own theories about children who lose a parent around the age of puberty. Take that with a grain of salt because I'm not a psychologist, but it's based on my own observation and experience.
Believe me, I'm not judging YOU. I respect you for doing what you need to do for yourself--I'm simply trying to do as you requested and I"m putting myself in your stepdaughters' shoes. I hope you see this as constructive.