This is technically a Tennessee question. In Kentucky, there's no cause of action against a person who is designated as medical proxy in making what turns out to be the "wrong" decision concerning medical treatment. There isn't even a cause of action against the doctor unless they actually do something negligent. There are many risks involved in surgery, so even the decision to have minor surgery should not be taken lightly.

If you decide to take responsibility for medical decision-making, make certain the hospital understands you are not guarantor for any medical bills. It is possible your sister needs a guardian appointed for purposes of applying for Medicaid coverage, etc. Speak with the hospital social worker for assistance in these matters and if there are attorneys at the hospital able to assist, all the better.

Ultimately, you need to do what you feel is right, and it sounds as though you are very uncomfortable making such decisions for a virtual stranger. In which case, feel free to refuse. Those who know her best are in a better position to make those decisions, they would know far better what her wishes would be. Even the estranged husband would know more.

This is exactly why advanced medical directives/living wills are extremely important. Visit Attorney General Jack Conway's website for sample forms, and make copies available to your physicians and family members. That way no one is ever left guessing about your wishes.