Employers typically do not wanter employees to discuss wages amongst each other, so they make policy to prevent it. People are also fearful of comparing, fearing that they may make less than their counterparts. There are employee disputes when they realize the differences in pay, like a lower paid employee getting upset at the difference in pay. The appropriateness of the matter lies between the daughter and mother, especially intention of mother regarding monetary status of daughter. I, personally, don't care and will tell anyone around me how much I make, it doesn't matter to me as much as others.
Since most live short lives on this planet, there's little point in how much money I make comparatively, as I see it - I can't take any of it when I die. If there's a mother's agenda, then it's often up to the daughter's reasoning to discern why it's important to disclose monetary ordeals.