Maybe for starters you could make one beautiful all white pillow, with lots of lace and white embroidery or some such thing(perhaps a ring-bearer's pillow and keepsake for the bride) and check to see if she gets excited about that. I see too many posts on here from people who have knocked themselves out to make a special quilt for someone only to feel let down by the person's response. If they have no personal experience making something by hand, some people just don't get it. Maybe the bride is telling you that a quilt, as many non-quilters imagine them, is not something she hopes to own. Unless you had evidence that this soon-to-be DIL might really hope to have an all white handmade quilt, I think I'd check her gift registry for ideas and focus my quilting efforts on someone who will definitely appreciate them. It's much better than putting in a lot of effort only to receive a lukewarm response.