I'm sorry, it's difficult to deal with old age and infirmities along with money problems.
Some of the solution would be for the sister to earn money as well as the daughter. But there is probably something I don't know about so it's just a shot in the dark on that.
I and my sister, are dealing with old parents that are mentally going downhill. Almost a year ago, we started a cleanup outside the house. Most of the work was done by the "apostate" side of the family which included my sister's husband. The other side wimped along.
I think on a human level its good to help, but only give what you can and that can mean different things to so many people. I would sit down and figure out what is reasonable.