What do I do when a younger brother, having seen our mother only 3 times in 5 years, takes over her cancer care?

I too am sorry for your loss but agree with freqflyer. I lost my Mom in Sept. She went thru what you are describing. A doctor had to order Hospice. An RN comes in and evaluates the patient. If treatments would no longer help improve life and patient was terminal, Hospice keeps them comfortable. If in extreme pain meds may make patient out of it. Most meds are discontinued except when for comfort. In final stages the body starts to shut down. They Can't swallow so Can't eat or drink. My Mom shut her eyes and never opened them again. Sure she heard but wouldn't open her eyes. Took her almost two weeks to pass. What happen to Mom was normal. You Can't blame ur brother.

Denied, so sorry for the passing of your Mother. I realize when a love one is seriously ill it can be very emotional for family.

Please note, cancer [depending on the Stage] can take a major toll on a person. Sounds like your Mom's body was shutting down. No way to stop that. When that happens, a patient just cannot eat or drink water because the stomach stops digesting and water would just sit in the kidneys, thus very painful.

Also no one goes onto Hospice until the Hospice doctor declares that the patient only has a short time to live.

I've never seen "dehydration" as the cause of death. May I ask what was the cause of death on the death certificate?

Was your brother your mothers medical POA?
Is he the person she chose to make her medical decisions if she was not able?