Good morning, jcorll;
JPD has probably the most appropriate insight on this for you. Noting that the Kodak Retina II is a camera that is associated with your grandfather, get it cleaned, lubricated, and adjusted to fully restore it to normal operation. Then use that very camera to take some photographs of your grandparents. Some "formal" posed photographs, of course, but also some that show them in settings where they are doing things they enjoy; with their hobbies, their pets, their car, in the kitchen, for example. Do it soon, while you still can. Have some nice size prints made and put them into albums. Present one to your grandparents, one to your parents, and keep one for yourself.
That camera can be a way to help strengthen the relationships in your family, to promote some nice discussion, and to create some great memories. Just do it now, while everyone is here. You may be surprised at how fast things can change, and you can never get them back.