In this case I think you should do the work, add it all up and make a bill.

When you give them the finished work, you give them the bill and tell them they can pay what they can and tell your darling wife that she will have to make up for the remaining bit.

No, seriously. This is a gift from your family to that family. But you are ok to show your wife how much she has given to them by showing her the bill. And if they ask what they can do for you in return, well, perhaps one of them can do something for you in future.