I don't think its too sarcastic. If you feel it is, maybe taking out the grandchildren comment?

Stressing the importance of current expenses is good. Maybe you can get proof of what is spent/needed? Something outlined showing what you and your hubby spend vs. the $14 she does.

I'm surprised they haven't attached her wages. My dad had his attached and my mom finally got all that was owed to her that way.

Good luck with that. Its a shame people don't feel they need to do their part when it comes to raising their own children.