Help, my mother has dementia and is also the absolute nastiest person I know. She thinks she is fine and refuses any help. Any ideas?

To begin with, my father became very demented 5 years ago and the first thing my mother did was get herself appointed POA they live in Mexico and when mom did this she made it so no one else can ever be appointed for anything or make any changes as long as she lives. Not only do they live in mexico they have never been legally married. My sister has tried to help to no avail. Mom is now becoming very demented but will not accept the fact and becomes hostile if one of us bring it up POA or anything, else to help, dad was very well off but can make no decisions at all mom has been secreting his money away into her accounts all over.  At this point she takes half of this pension check every month, it disappears.  She forgets, over pays, pays 2 or 3 times the same bills and just gives money away like crazy but it's always dads money and we fear she will run them both into the poor house. she always says she is broke, and hides all of her money and half of dads. We are concerned about this because they are not married. I wonder if there are any laws in place in Mexico to have someone appointed guardian or something like that before it's too late. before  To make matters worse mom is really the most narcissistic nasty person we have ever known. thinks she is totally fine and will live forever she is 86 and dad is 91. Does anyone have any suggestions about what can be done here in Mexico? Mom is getting worse daily.  thank god the nurses she hires are honest because she pays them and 10 minutes later forgets and pays them again, sometimes 3 or 4 times. the nurses call and tell me and return the money and I put it back in the house expense account.  She just can not be dealt with and get angry if we call her on any of these things.  Does anyone have any ideas for this impossible situation?