If it was me, I would very nicely say "you know, someone asked me to make them the same quilt that I made you, do you care if I go to your sister house and pick it up so I have the pattern?" That way you get it back and never mention it again, after a while she will forget about it and If it ever comes up again, just say oh .... At the same time she will know that you know its at her sister house and what it is being used for. That is if she is paying attention. This way no one gets into a fight and the PEACE is maintained