I'm sorry you're going through all of this!
So much of it resonates with me. For example, my brother also blames all of his problems on my parents. (Apparently, I had the same parents and slept in the same room with him on bunk beds and just somehow have forgotten all of the abuse!) He also continues to use them in any way he can. It is absolutely infuriating! I agree with Adeline about limiting your communication with her. I understand that you want to maintain your relationship with her because of your nieces, but I wouldn't call her for a sympathetic ear. I would keep conversation as superficial as possible. Even if she's clean, she's obviously still exhibiting addict behavior.
As far as your mom and dad's situation - I relate to that, too. I'm the one who thinks she can go off of her meds anytime she feels good. lol. I think that dealing with us who are seriously mentally ill probably requires a lot of the same concepts as dealing with an addict. No one else can control it or cure it. There's a reason they send you to rehab for suicide threats/attempts. My husband talked with my psychologist a few times and it was really helpful for him/us. I hope your dad reaps positive benefits from counseling, too. If nothing else, I'm sure he needs some time to vent after being under so much pressure for so long as a caretaker.
Best wishes to all of you! I hope things soon improve!