I'd just say that I had too many projects already backed up (which is true), but that if I ever got to the bottom of the pile, I'd let her know. Just because you make quilts, doesn't mean you have to make them for everyone who asks. Relative or not, it's gutsy to ask when the relationship is not a close one. Unless, of course, you want to charge her plenty for your time and work. That's a whole 'nother story.