My Mom has vascular dementia, but is in serious denial that anything is wrong with her. My sister tried to take over the mail/paying bills but my mother wasn't having it. So my sister has to painfully babysit her for bill paying. Mom has a history of hiding things, even before the dementia, and puts her check book in various places and can never find it without a search even though my sister put a pretty box in her kitchen for her to keep it.
Mom never knows the date, doesn't know the names of the vendors to which she owes money (every week: what is Eversource? Its your electic company Mom), struggles with filling out a check correctly, doesn't remember what was and was not paid and is completely confused by the whole process. But refuses to give it up because she thinks my sister will then steal from her. My sister has informed delivery so she knows what arrives at Mom's at least. Mom got a couple of refund checks recently from getting rid of cars but she promptly lost them that day, and denied that any checks came. Sister walked into one of the bedrooms and found the smaller check on the dresser and showed Mom. Mom still refuses to believe that there was a second check which is still missing.
Mom hid her will, which we can no longer find, and so sister had a lawyer draft a new one (the original will was from a long since deceased lawyer). Mom refuses to sign it. Even when I read her the letter my Dad wrote to my sister and I (executors) explaining what he wanted and why. She'll agree that yes, that sounds reasonable, but when any authority figure is around (aka lawyer) she freezes up and refuses. We did badger her to at least let my sister have power of attorney, but we really had to bully her for that and I felt bad even though it is so so necessary.
Dad tried to wrap it all up neatly before he died, but it hasn't turned out that way.