Hi, Barelysmile~
I noticed that you mentioned that you have the inclusion of your 20 yr old son living in this arrangement, as well? Did I understand that correctly? If so, I was just curious what contribution he provides in this situation.
With the fact that most means of your resources are exhausted, which of course leaves you few options, therefore most of your time is spent in that house and in her company, (being around each other too much), perhaps volunteering your time in something which you enjoy might provide you some great relief.
It'll get you out of the house, provide you (both) time apart to breathe, allow you to nurture your own self FOR self, open and maintain any windows of opportunities.
Also, there are organizations within your community which provide free counseling and support groups which you can attend to seek emotional support or to improve your chances for some kind of employment (either through some sort of employment placement or employment training courses). All are free.
I have a family quite similar, so I can personally relate to some of your struggles. In my past there was a time where I had faced a situation where I had to choose between family or transiency, and sadly (without second thought), I chose the latter. However, I had no kids then, which, naturally made the choice much easier.
I truly hope this situation improves for you and that your perseverance for self-improvement (despite the odds) will prove to be the positive motivation your sister needs, as well.
Shangrala