Rancher,
I am so sorry to hear about your situation, it isn't a good one. I do empathize with you in many ways, my stepdaugther's were 5 and 7 when I met my wife 20 years ago. Their father was/is a violent alcoholic, the main reason why my wife left him. I also empathize in the fact that I've been around alcoholism my whole life, Dad, little sister, brother-in-law, etc. When dealing with this disease, a lot of cloudiness surounds the answers we are looking for, it is never easy.
Here is my opinion of your situation. I understand you are leaving, that decision is made. That you can control. Your wife (or ex) and her drinking, you can't control. How it will work with your stepdaughters, you may have to talk to a lawyer. Legally, you may not have any ground to stand on, seeing in the fact their biological father is alive.
A bit of advice based on my experience with my wife's ex. Whenever we went to get kids or had to deal with him, as much as I wanted to kick his butt a few times (grabbing at my wife, pushing kids), I had to maintain. When the kids would call us on his weekends and say he is drunk, the first question would be is he driving or are you somewhere other than his place. If they were afraid he would hurt them, we would get them. If it was his drinking that scared them, we would get them. He eventually realized that his drinking was pushing his kids away. If anything, he stayed sober when he had them.
The last piece of advice. Kids grow up fast. They also know more than you realize. I always tried being steady with my girls, though they aren't mine biologically, they are still my girls, as is my granddaughter (no biological kids of my own). When my oldest daughter asked me to walk her down the aisle and not her own Dad, I knew I did right for them. The youngest has asked me to do the same. Also, the girls do not drink, I think they saw what it did to their own Dad and made their own decisions/conclusions.
I know your kids are younger and I wish there was an easy answer. Just be a rock for the kids and be as involved with them as much as you are willing to and as much as you are able to. I hope this helps and my thoughts are with you.