The two posts suggesting some type of hotel or similar long term are, at the core, valid and sage. I mean, sure, on a surface level, it sounds utterly unsavory, but let me tell you about my ex husband's experience with boarding. It has actually been rather positive. Those that live in there actually watch out for one another. There's a woman there, about my age, not totally down on her luck or anything but the affordability of the rental market stinks to high heaven. She needs to take public transport into another town for work, but she's also been able to save up. There is a sense of security. For my ex, it has been good as he has gotten to know an elderly neighbor that he watches out for, even contacting elder services to make sure he receives transportation through them as opposed to walking to the transportation depot. Those managing the office ensure he gets his mail. There's a security guard/maintenance manager. Things like that. But it's cheaper than rent.
The point is in exploring options and not being expected to be your sisters keeper.