IMHO it would be worth your while to get some older quilts at tag sales or whatever just for the fabric. Then you could cut some fabric of close color or pattern, fold under the edges and applique them over the worn out spots. The pieces that are missing or worn out are because of old dye that weakened the fabric. These are treasures of your family heritage. If you want them to last much longer you should do something to help preserve them, including patching them if you can. They are gorgeous, and if you helped them out some, they would last a long time! Enjoy them...they represent a lot of work by your grandmother.