Should I help my mother, who has Alzheimer's, change her will?

I have been my mother's caregiver for about 20 years now, since Dad died in 89. My mom has been 98% wheelchair-bound for the past ten years due to a leg amputation. Her health has declined over the years and she now is battling dementia/Alzheimer's and severe osteoporosis at the age of 91. I have two sisters, one in-town and one out-of-state. I am the youngest. I work full-time. My sisters are both retired. My problem is that I get no help caring for her. The sister that lives in-town won't help, and the sister out-of-state says she wants to help, but really can't because of being so far away. I have discussed this problem with my mother numerous times. She realizes that I do everything for her, and that my sisters aren't helping at all. She thinks it isn't right, but doesn't know what to do about it. She has said several times to me that I deserve to be rewarded for all the years I have devoted to caring for her, and that she wants me to get half of the estate and the other half divided between my two sisters. Her will was drawn up 20 years ago, and was set up to have the estate divided equally between the three daughters. A lot has happened since then and now it sounds as if her wishes are changing. The problem is she doesn't know how to go about getting it done. She asked me to change her will for her. I told her many times I couldn't do this. I have always handled everything for her throughout the years and she doesn't understand why I can't help her. That is too much for her brain to handle. She can barely remember to do the simplest of tasks without help. She really doesn't understand. My question is should something be done? And, if so, what?