I am sorry for the loss of your father, and the fact that you are having to deal with the histrionics of your mother. who despite being 92 appears to be in full possession of her faculties. She is punishing you, and anyone else who tries to help, for her grief and there is little you can do to alleviate it until she allows you to help.
You are doing everything you can, practically and emotionally, but I have no doubt that she has a will of iron, and a determination to have her own way, no matter how detrimental. I saw it with my paternal grandmother and my father, both 94, not consumed with grief but fury that their independent life needed support, also with a friend's mother who lived to 104.
Do not feel guilt because you cannot persuade your mother to accept help; do everything to reassure yourself that she is safe as possible, as your brother is able to do, and maintain contact, but refuse to shamed by the 'you don't know how I feel, leave me alone.' Just remind her gently you have lost your father, and end the call, but maintain daily contact for your own peace of mind.
So sorry.