I think it's disgusting that the only way they'll Associate™ with you is when it involves them getting money from you.
If being part of the Christian Congregation of Jehovah's Witnesses™ makes them so frikkin happy, maybe they should try to get some money from their Loving™ Brothers™ and Sisters™, instead of from their mother, whom they are supposed to shun as being in league with Satan the Devil™.
I'm really sorry that they are using you like this. You deserve so much better. If you can, try to find a way to stop rewarding their psychological abuse by giving them money when they want/need it, and insist that if they are going to treat you that way unless there is money involved, that you cannot continue to give them money. They either treat you properly ALL the time, or your purse gets shut tight.
Besides, how do you know that they aren't putting part of the money you give them into the Contribution Box™? Do you really want to contribute to the WTS, even indirectly by financing your daughters' JW lifestyle?
Make no mistake about it: Just because JWs dress up nice and smile pretty and don't do drugs, what they do to families is thug warfare, plain and simple.