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LW, I haven't got a clue how to advise you, but I feel it must be awful what you are going through. I forget whether you mentioned you have a good therapist, or not, but they're trained to help people deal with the illnesses of family members and if you haven't already, you might want to give it a try.
One more thing: I think Cary may have forgotten to address the issue of the way this situation is hurting your relationship with your sister. If I had something like this getting in between me and my sister I'd be doubly miserable. I wonder what advice posters may have to you about that.
Take care; this situation sounds totally miserable and I hope you are finding some ways to get away from it sometimes and enjoy your life anyway.