If you think it would be appropriate, just let your aunt know that if she ever decides to get rid of it, and if none of her children want it, that you would love to have it someday because you would like to see it remain in the family. Your excitement at seeing it, documenting the history, etc. will convince her of your love for it and let's her know you really appreciate what this quilt means. If you are close with your cousins, you could let them know that if none of them want it, you would treasure it. I think saying it this way let's her know that you recognize your cousins would be first in line for inheriting the quilt, but if none of them would appreciate it and your aunt loves it as you do, she will then know who will really cherish it.