You tell her that you can no longer help her.

I have no idea what you do with the house - it seems like that is too big to just write off and let her live there, wrecking the place. Who is the trustee in charge of the estate? If there isn't one, maybe you could hire a lawyer to start taking care of these things. If the lawyer works for the estate, then the money to pay him/her would be taken out of both your shares, not just yours. Maybe they could start taking rent out of your sister's share as well -- and include the money you've spent to pay bills to ensure the place remains salable.

I am not sure how you can stay on good terms with someone who expects you to bail them out on a regular basis. I'm sorry you are stuck with that no-win situation.