You are very gracious and I am sure your SIL and daughter will appreciate your help. I imagine the fundraiser will be more likely to succeed if there are some fresh, tantalizing cakes instead of the kind of deflated, lumpy efforts made by those who can't bake (I speak from experience here!) The good karma will no doubt find you and come back when you really need it. It says a lot that your family feels comfortable enough to volunteer you to help them out - they know you will have their back and you just can't put a price on that!