My father had dementia for several years. Worst disease ever. My heart goes out to you and the others dealing with it.
My take on responding to your mother's anger: Don't argue, don't reprimand. Just redirect/distract her as much as possible. If she's anything like my father, the anger will blow over really quickly anyway.
And (hard as it is) don't take it personally. At those moments, try to think of her more as a patient than your parent.