Too bad things like this get in the way of family. When those lovely folks who are celebrating 70 years together are dead and gone and no photographs are made what will everyone have? Hard feeling to remember? I would try to do all the groups you mention. Sometimes grownups can act like children. If you can't make pictures of them together, get them separately. You can scan the negs and put the separate people/groups together in photoshop. If you're not comfortable gettting this task accomplished, serving as a referee, find someone to help you; you may need a bad cop and a good cop to get things in order. I've had a few of these but I have worked them out.