Are your parents first gen immigrants by any chance?
I ask because mine are, and this is something I've struggled with. In my culture, it's socially acceptable to ask someone what their salary is - even if it's a friend or acquaintance, never mind a family member.
In fact, I have repeatedly had to remind my DM that it's completely inappropriate to ask my English DP about his finances! She doesn't see it as being nosy or invasive, to her it's just "basic" information that she would expect to know about her daughter's partner. It is infuriating as hell, but I have to remind myself that she genuinely doesn't understand why it's inappropriate.
There is also more emphasis on the collective and the family unit, so often money is seen as "shared" or "family" money - this often comes with an expectation to help relatives if you are financially more fortunate.
To be clear, I don't like it at all - I much prefer the British approach, and completely agree that a) it's none of your family's business and b) they have no claim on this money. However, IF it is a cultural thing, maybe this changes how you approach it with them, and how you can get through to them?