How do I get neice to realize caregiving for her mother with dementia is so rewarding and fun and she has to?

My sister has always been very persnickety and very ocd. But otherwise a nice person. My niece said she was neglected and abused by her mom and the only reason she turned out ok was her best friend and late husbands mom let her live at there houses. According that's she old she is impossible. She isn't that bad. My niece is dramatic.

She's allegedly physically attacks my neice. She rips up my nieces things and screams at her all day. It's part of caregiving. I am sure it's not that bad. It's such a gift of sacrifice she is doing this. My other sister said she needs to suffer because then she will get into heaven.

My neice doesn't want to come to Christmas because I embarrassed her by telling everyone at the restaurant about it.

Also she needs to work and her mother needs her constantly. her mother makes her beg to have food q tips bottle of water toothpaste and to shower and wakes her up screaming at the top of her lungs.

it's a rite of passage. I don't understand why my sister doesn't give her money freely. She is worth at least two million.

Updates
5 mo
I just thought it would make her feel better if I told all the waitresses and talked really loudly about it so everyone knows what a great person she is. I have been so sad and emotionally distraught over this.
Updates
5 mo
My older sister is being cared for by her. My niece is 39. My sister is 77. She has other kids. The one is in prison for life and the other one went no contact.
How do I get neice to realize caregiving for her mother with dementia is so rewarding and fun and she has to?
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