I am not who you asked, but I don't know much about my parents' finances, either.
If I had access to that information I could make educated guesses about how to care for them as they age. Are they able to retire? How long can they live before they are out of money? How much do I need to have saved by the time they are X years old to ensure they are cared for?
Unless I was going to let them live on the street, it's something I must consider.
Well, more than the salary (which I do suspect is low), I would like to know her retirement/medical budget since she's heading for retirement and her family has a history of very heavy medical issues (Parkinson's and Alzheimer's).
This is why I'm especially skeptical of her assertion that is "has nothing to do with you." It _does_, because if/when she needs help, of course you're going to want to be there. Still I can't think of a way to say "It is my business because it's possible that one day you will have dementia and I'll have to pay for someone to care for you all day."
Sure, the assertion is fallacious. Pointing that out doesn't actually change anything, though. You can't force intimacy (and sharing otherwise private/sensitive information can definitely qualify if you feel that way) where it's not welcome. The 'problem' (if you see it as one) is that sdrothrock's mother doesn't see them as an ally on this front. That's what'd need to change.
Just curious, what would you do with that information if you had it?