A couple of suggestions. First, you must get your hands on a copy of Roz Chast's brilliant illustrated book Can't We Talk About Something More Pleasant, about her own experience of aging parents.
Second, if your dad can't convince your mom to be a little less dependent, maybe you can conspire with him to get him to create a binder with all the information she would need if something happened to him. Which accounts get paid automatically, the passwords and URLs for any important websites he uses, who manages any investments, etc. One binder -- all she has to know is where to find it if she needs it, or if you ever need it because he's become dangerously forgetful.
My own father (82) is extremely prickly about topics like this but what has helped him to calm down and take some action is that suggestion - that he create a binder with all the important information, financial and otherwise, that my mother would need if he "got hit by a bus" (I say it that way to make it seem like his demise would be an unlikely event instead of the most probable outcome in their situation.)
My dad gets fiercely protective regarding their finances. He put me in charge of everything in their estate planning (they have four children, but it's all me) yet he doesn't want to tell me anything about anything. Suggesting it to him as a private project that no one but my mother would ever see helped him to get it done without shouting and getting furious.
At least, I hope he got it done.
