I'd focus on the following seven talking points:
- Unfortunately, life is not fair sometimes, and we don't have control over that
- Just like we need to keep our bodies healthy, we need to keep our minds and hearts healthy, to be a total person
- You see some things in their mother that are not healthy, and are making you sick, and you need to separate from her to keep your health
- You do not want to control their mother, but think she is making choices that are unhealthy for YOU to live with, so you need to separate
- The girls are like your own daughters in your heart and mind, but unfortunately the law does not recognize this, so there are things you would like to do for them about this situation that you legally cannot (this is where the life is unfair part applies)
- You plan to live separately, but this does not mean you love them less or are leaving THEM, and you will be available to them if they want, and their legal family allows, up until legal age, when they are free to come to you by their own permission
- They probably won't understand some of this now, but most likely will in a few years
When the detail questions come, have a generic statement ready like: "Your mother drinks alcohol in a way that is unhealthy, and worrying about this makes me unhealthy. Since I do not control her, I must do what is healthy for me, because the last thing you girls need is two unhealthy people as role models."
I'd stay clear away from any statements about wanting to take them away from their mother, or their father, or any statements about their parenting qualities. As soon as you open this subject of leaving, you essentially become a concerned citizen for their welfare, not family, (from a legal standpoint of your "rights" or lack thereof toward your stepdaughters) and must construct your statements likewise so as to not entangle yourself into any legal corners.
Just MHO
CLMI