Narcissist with Dementia and Domestic Violance

I have grown up with a narcissistic father constantly mentally and emotionally abusing my mother and us as kids.
I've cut him out of my life completely but it also came with a heavy price my mum.

She wants to stay in contact with me there's no issue with her wanting to, it's just the control he has placed on her.
I've moved interstate to get away from it all, but sometime before I left I said to her she and my mentally ill brother can live with us. And we would support them until such time they can get on their feet for themselves (which could take years) but my husband and I were fine with that. Instead she just dismissed it I offered and offered but still no even though both her and my brother are suffering extreme mental and emotional abuse.

He's so controlling to the point my mum isn't aloud to call me he checks the phone bill every month. She might get to contact me at best once a month sneaking a call on my brothers phone when he goes out.
I really worry about them I believe he also has the start of dementia he drinks 700ml bottle of spirits a day and there's no end to the relentless abuse he throws at her and my brother.

I love my mum I feel hopeless about the whole situation I get upset after every call although I hold resentment and anger with her for not being stronger and leave his arse. She chose her husband over her kids a long time ago she rejected me and my offers to help. I stood up him on behalf of the whole family because there is no one that likes him they merely put up with him to see my mum without back up everyone is that afraid.

I'm heart broken I don't have the support and love that a mother gives her daughter when she starting having children, my kids are missing out on their grandma but the protection of my kids from that evil human being is paramount.
He believes that my kids are bastards because my husband and I weren't married when I had them.
All I've ever wanted is my mum to be free, she's a prisoner a slave to my father.
I guess my question is... If there is anything left that I can do?
And if there isn't how can I move past this for my own mental health?