I would simply say that you are already so over-committed with your own quilts that you would not be able to make them for 2-3 years or more, or simply state a figure that you know they would not pay, although they might surprise you. I understand your reluctance since you are not that close to your step sister, as I only make quilts for a handful of very, very dear friends and our bi-annual family reunion quilts. Just don't be afraid to say no.