I'd say, if you can afford it, just send it on with your good wishes. You already know you won't get paid if you send a bill, so why bother. It is only going to create hard feelings from a) the SIL and perhaps b) the niece with whom you have a loving relationship. I doubt you would want to jeopardize that relationship. Again, if you can afford it, chalk the expense up to just being one of the good guys and let it go.
If finances are particularly tight, you could always include a nice note saying that it would help your situation with a contribution to the cost of X dollars spent in completing the quilt vs an actual bill.