Hugs and supportive thoughts coming your way. There are lots of things I am thinking about that could be helpful. You certainly have a lot to deal with, sounds like a stressful situation.
Here are a few things that I am thinking of:
1. Be on the same parenting page with dh (discipline etc.)... there needs to be a clear message about expecations, boundaries, etc coming from BOTH of you (for many reasons... ).
2. Be good role models... make sure you and dh are NOT saying anything negative about the ex....there might be enough bad things to fill many novels, but sometimes it's just best to zip the lip--they will learn from their own experiences what kind of mother they have. I"m not sure how I would handle the ex's lies about the abuse stuff... I would leave that for a family therapist to handle.
3. Keep and make your boundaries to minimize the chaos. If the ex is threatening the stability of the girls, then don't go out of your way to include her in what your family does. I'm sure there are good reasons for wanting to do this, but consider what things you coudl limit or change to improve the day to day chaos that arises. You don't have to act like the family is one big happy family when it's not... I think it's great what you've done, but it doesn't sound like she's going to return the favor.
4. Be realistic about what is within your realm of control... you can't make up for what the other mom did to the girls by leaving. You can't make up for how she behaves now. You CAN be yourself--a loving, nurturing, caring step mom who models healthy, mature behavior.
5. Get the courts to help if they are able... it doesn't soudn like there is much time for the courts to be involved in this situation with the younger girl and the older one is already matured out of custody. SOmetimes the courts could mandate a parenting class, or something that could be useful.
6. GET SUPPORT--I think you are doing the right thing to come here to MDC... I'm sure you'll find lots of support. Check out any local resources in your area... there are usually parent resources available through a variety of non profits or churches or hospitals... maybe they have one for step parenting... or at least parenting teens.
Hope some of this is useful... this isn't my area of expertise, but I have had some experience in this area. Good luck, and I would also try posting in the parents as partners forum or the parenting teens forum for suggestions about any specific questions you have.