My mother has always been a nutcase so we all just ignore her now
But I'm seeing it with my Dad as he gets older. He's an extremely sharp man and at 68 still does consulting work and teaches classes at the local university. But I see his short term memory starting to go. Little things that probably people who don't know him well don't even notice. But I'm just so used to him remembering everything it freaks me out.
I'm trying to get him to tell me as many "old stories" as he can now so I can get them in writing and pass them down and thankfully those he remembers and even more important, he loves telling them.
His driving is starting to scare me too as he has a sports car and his reactions just aren't as good as they used to be. Thankfully my stepmother is picking up on this and offers to drive most of the time. I still constantly worry about it though.
His Mom had Alzheimers and I've heard it's hereditary so I may be a little paranoid and picking up on things that are minor and I see as major - who knows.
Best thing I can suggest to be sure she has all her legal affairs in order, living will, possible POA for you, try to get her to discuss what she would prefer if she gets to the point where she can't take care of herself, etc. I wouldn't stress too much about your Dad's stuff being there - it's a comfort thing for her I suspect.
And be sure to tell her you love her every time you to her and do what you can to end each time on a good note and not a bad one - it will make things easier on her and easier on yourself if the time should come when you can no longer correspond with her for whatever reason.