What do you do when your aging parent is narcissistic?

I've spent most of my adult life trying to get away from my abusive parents. Though my father died almost twenty years ago, my mother is still living. Now, the day I have feared for so many years has finally arrived. My step-father has died leaving only me to care for my ninety year old, relatively healthy and definately mentally sharp, mother. 

Though it sounds normal and responsible for the only daughter to care for her aging mother, what responsibility do I have to a person who has been emotionally abusing me my entire life?  Seventy years has seemed like an eternity as I have endured the pain my mother has inflicted. For the last couple of years I have stepped back, and my health has begun to improve as I have been relieved from the stress of having to deal with her. 

I have dreaded this day and hoped that she would die before her husband, or that someone else would step forward to deal with her, but there is no one else. There isn't any other relative to help. The people she knew have died, and she never had any friends. There is no one but me. I've been struggling with the urge to just run as far away and as fast as I can, but being a very loving, responsible INFJ (introverted intuitive feeling judging) person, I decided instead to do the "right thing".