You are not obligated to pay for someone else's surgery. If I were a betting man, which I am, I would bet your aunts heart problems are directly related to a lack of exercise and bad diet.  

Could you loan the money to her? Does she have property she can add you on as collateral?

I would throw it back at these family members to add you to the deed of their property and you would loan them the money. It can get a little complicated, but lets see who will put their money where their mouth is.