Does this bring back memories. How well do you know their Dr. If well, ring just before their next visit and tell him your concerns. Especially regarding help around the house. He can, in his own way, recommend they get help until your Nanna picks up as this will help her get better quicker. Once they start getting the home help, they realise after a bit that it does help and you go from there. With your concern re driving it is worth letting the Dr. know as he would be upset to know about his driving ability. Can you go to the Drs with them and go in with them? I started doing this and was amazed at the things I heard. We are fine usually meant something different. I also went through the same thing with the ACAT and while she was there asked her to do both.
All the best and make sure you look after yourself and don't get run down.