Hi Van,
As hard as this sounds I would wait, sounds like you have beat yourself against the rock to many times already, my 64 year old father was just having the same conversation about my 83 year old grandmother and my dad's 55 year old brother, he is once again back at home with them and she is locking her door at night yet always has to bail him out, even after all the theft and abuse. We all understand that at her age we will never get her to see she is an enabler and sadly when she passes he will probably end up homeless or dead.
First thing I would do is pat yourself on the back, enjoy the fact you are not the burden your mother carries, try and have the best relationship you can with her, in our case we do not visit my grandparents when my uncle is there, and I do not bring up the topic anymore and I believe most of the family does not either. It is hard, and it sucks, and you want to grab your mom or grandma and say " Stop it" but I have found that reality is much like the user can not admit or see they can't help it anymore, an enabler this long and this deep can not see it either and they can't see that it is not their failure as a parent.
Enjoy your children and your life, if your mom is not open to therapy or al anon or the ideals of how she is enabling then all I can see you being able to do is picking the best moments possible to visit with her and make the best of what you got, but I would halt your sisters damage in your life for now, I know that is not easy, it breaks the heart and soul, but I believe you need to step back sooner rather than later before you hit the point of no return even if she does stay clean long term someday and you will not be able to reconcile even then because of to many disappointing chances you have given her. Stabilize yourself first, no matter how hard we try it seems the worry and angst we have for the A in our life will manifest in our daily lives and affect others around us, and in the mean time see what time hands you to work with is my opinion.
Stay strong, you probably heard this before, but you didn't cause this, you can not control this, and you surely can not cure this! I am sure everyone on here wish they could though, because it sure isn't easy on anyone. I wish you the best on everyone's journey in your life.