I really agree, I'm afraid. Neither you nor the kids can compete with booze and boozysisterlove, and it's foolish to even try. Let them go on a country boozefest together. Advise them that he'll be spending every evening, and all of the nights, at hers, and have a heart-to-heart with MIL if you're close. Change the locks and harden your heart.
Loads of doctors are alcoholics, addictions aren't fussy! Please DO read Al-Anon's publications, and maybe give them a ring as well. You need get your head round what's happened to your family, and the addicts are the last people in a position to explain. I feel for you :(
Some Mumsnetters adhere too strictly to the principle of letting your addict reach rock bottom, withdrawing all support. Having watched families sentence their loved ones to death like this, I'm not such a big fan. Your husband, however, has a built-in soft landing with his sister. I think that's a good enough reason to bring out your big guns early. Talking to Al-Anon will help you understand why, I hope.
Take very good care of your self. Gather support around you, and don't let your DC assume their dad's behaviour is normal or acceptable.