How do you stay in your parent's life when one sibling has taken complete control?

My brother has taken over complet control of my mother's care. He is her medical proxy and POA (done without my knowledge). Everything has to be done his way, and wonders why I only visit monthly (I live a distance away). When he needs help he is is demanding and dictorial in the way he wants something done. He is not open to any suggestions, and is verbally abusive when he does not get his way.

Unfortuantely, my mom lives with my brother. I have been told by a social worker that the relationship is co-dependent and unhealthy.

My mother suffers from dementia (which my brother denies) and has is now recovering from hip surgary. I love my mom, but feel pushed out of her life.

I do not have always agree with the care she is being given, but have no say. I am tired of my brother's abuse but don't want to lose my mother because of his actions.