Quite a common problem, parents demanding money.
I have dinner at my parents 4 times a week, my mom always cook in
excess of what everybody can eat (also a common problem). One time
tried to drop hints that the money I give to her is insufficient.
My first and only response, cut down the food. The money issue
never came up again. I give to my parents more than a grand a
month, which includes the 4 dinners per week for my family of 4 and
lunch for my two kids each work day. This excludes money given by
my other sibling who also have similar arrangement as mine.
My point, is, yes filial piety is important, but parents should not
abuse this. I am aware the sometimes treatment between siblings is
not fair, but that's life. Occasionally we need to put our foot
down even if it means offending the other person. Dam if they
understand dam if they don't. I know most of us try hard
enough.