You could very well risk damaging the fabric as over the years if the top wasn't impeccably stored the fibers weaken. I know you'd really like to quilt it, but it really could be a gamble in this situation. Without having the top professionally appraised and looked it to determine it's stability and integrity, you could really risk losing an heirloom to your family. In my honest opinion, I think you'd be better off by cherishing it as a top only from a beloved family member. Truth be told, not all quilt tops require and/or need quilting in order to be displayed and enjoyed by everyone! I wouldn't risk it personally.
Explosive blessings, abundance and inspiration to you all!
- The Creative Seamstress