My Father died three months ago. Unfortunately he had nothing in place for my Mom who suffers from Dementia prior to his death. There are three of us siblings. We have managed for about 2.5 months to work well together, rotating staying with Mom and making sure she is eating and keeping the home up. We even got Visiting Angels to come in 3 days a week for a 4 hour shift to give each of us a break during our time staying with Mom. What is going on now is my younger sister has been acting irrational and overprotective with my Mom. She has quit speaking with us and only e-mails us when she want to tell us what she wants. How do I stop this aggressive behavior from her without destroying the fragile relationship we have now?
If things get really to the point it is harming your mom, you need to go to court and have someone appointed Guardian. That will solve a lot of problems over who is in charge. It may alleviate your communication issues to have a neutral 3rd party, experienced in dealing with vulnerable adults' needs, appointing someone. They will usually choose someone geographically closest, who is able to spend the time.
Good luck. Some day your sister may "get over" whatever it is (and it may just be grief) and I hope you will be ready to listen to her. She isnt the Devil.