BIG HUG I am so sorry you are going through this and that your mom is having a tough time right now.
Is your mom open to counseling? Everyone always thinks they are doing better while taking their medicines and then think they can come off because they are better when in reality you feel better because you are on the medicine. It took me a long time to grasp that seemingly simple concept. urgh.
Your sister is acting like a typical addict (I would know because I was one). There is no reasoning with her and even though she lives there she is never going to be any help since she can only think about herself right now. It is a sad situation and I wish there was a magic answer but unfortunately she just can't be counted on emotionally, physically, or financially. Have you thought about detaching from her and putting all communication with her at a if you MUST category otherwise just leaving her out? Might spare you alot of frustration.