Honesty..always the best policy.imho.
At their age they already know something is happening,and most likely know that alcohol is at least in the mix. I gave my kids a copy of "Under the Influence" to read. I have always treated the alcoholism like any other subject (ie sex education) ....answered the questions factually and with as much detail as they wanted (that was approriate) and kept the lines of communication open. They know they can always talk about any questions,fears,concerns they might have. Their imagination is more frightening than the facts. I found the same thing to be true when my own mother became terminally ill. To know that there were not "surprises or lies being told" made it easier for me and my children have said the same thing,too.
take what you like and leave the rest.
Knowing the truth also will help relieve them of the burden that it is somehow "their fault",imo. Their father is ill but unwilling to seek treatment,is what my kids know about their own father and it has helped us all stop enabling him now that we are understanding that.
Good luck to you all.