Patience and love, peace and quiet---that's all you can do. As a step-mother of children with an equally or even more so crazy mother, I can empathize. My dh's ex told the kids that she and her new husband were to be called Mom and Dad and that they were to refer to the dad and me by our first names. When they asked me about it, I just asked what they wanted to call us. When they asked if it would be all right to call us mom and dad I said that would be fine. In fact, why don't you call whichever ones you're with mom and dad then it won't be confusing and you'll always be with a mom and dad. They liked that and that's what they did. Don't run their mother down to them, that only makes them defensive and leaves them with a feeling that they have to choose which "parent" to believe. Be honest, loving, have a few very firm rules and don't waver on them. This will set boundries that will form a comfort zone for them. And most importantly, treat them with the same degree of respect that you expect from them. You'll do fine. I have 4 step children and they all treat me with love and respect---the same way I treat them.
Once my step children asked why their mother didn't call or come and see them. (Boy was that an opportunity to tear into that woman) I thought about it for awhile and then asked them if they ever wondered why I was so insistant that they learn to prioritize their responsibilities. When they said yes, I explained that when their mother was young nobody taught her to do that and that right now she had her priorities mixed up. I told them that maybe one day their mother would realize that they were far more important than traveling in a semi with her new husband and would want them to live with her again. I also told them that it might not happen, she might never learn to prioritize but that she was doing the best she could to see to it that they had a good home by letting them stay with us. I told them that she was showing how much she loved them by making sure they were taken care of even though I was sure she didn't really want to be away from them.
okay, forget what I said about being totally honest, sometimes you have to fudge on what you really believe in order to make the children feel secure and loved by all parents.