Hey Bauhaus - it sounds like you've done a lot for your sister and I think she has been taking advantage of you; you don't deserve that. I also have relatives who are like this; I don't think they do it on purpose, but they (my relatives) manipulate people into supporting them because they can't take care of themselves, and then guilt-trip the supporters when they try to disengage. It's very abusive in a way.
bell hooks wrote a few books about love that I have found helpful, starting with All About Love: New Visions. I think reading that was the first time I had considered that "love" is how we treat people, not just something we say or feel. It's been awhile since I read it, but you might check it out - she writes about how abusive people will say they love you but then act in a very unloving manner, either physically or verbally. It's a small book and she quotes from some other authors, like Erich Fromm, who go more in depth if you want to pursue this subject for your own self-reflection.
I was going to suggest you see a therapist to help you figure out a strategy to part ways with your sister, which it looks like you are doing. I think if you give her a deadline - like 6 months - and offer her a list of some resources you will have been more than generous. You'll need to be firm - whatever you decide, don't give in, because that will just reinforce (in her mind) her beliefs.
It's true that life is really hard for older people in this crappy economy, but your sister might qualify for assistance for low-income seniors. If you want to offer her some resources for when you part ways, you might check local senior services - there might be some subsidized housing options for her, probably with a waiting list. If you do this I would suggest giving her a list of places to contact; make it clear that it's up to her to contact these places on her own. I think it will be better for her if she learns to ask for help for herself - it's been very helpful for me. If she's lucky she will be connected with a social worker who can help her develop a plan for being independent.
I'm wondering if your sister might have an undiagnosed or untreated mental issue - maybe ADHD? Poor decision making and not wanting to deal with difficult tasks/problems, especially regarding finances, are big flags. I was just diagnosed with ADHD at age 38, and learning more about why I have problems with the tasks that other people take for granted (like paying bills on time) is helpful. Of course since I've been reading a lot about adult ADHD I'm seeing it everywhere, so it may not be applicable to your sister. However if your sister does have ADHD and gets diagnosed, she might qualify for extra assistance - financially and help doing things like paying bills. It's up to you whether you want to suggest this to her but it might explain why she acts irresponsibly.
Whatever happens, you deserve to have a fulfilling life, and you shouldn't feel guilty about pursuing that. Good luck!