Consider this: give yourself time to heal from this very stressful period of time before you make any major decisions (like divorcing).
When my mom was dying of cancer, my sister was the one taking care of her. I know it took a toll on her. I am so grateful and appreciative of that she gave up and put herself through in order to help our mom. I assume that the sister who did call was appreciative of your efforts. They both should be. You were all effected by your mom and handling it different ways. I am glad at least two of you talked. My sister was the middle child. She is the least selfish of the three of us. Are you also the middle child?
You have been posting like crazy and on an emotional rollercoaster. Time for rest not action. <<hugs>>