One solution would be to change the dictionary phrase to be similar to that for parents and godparents and just list grandfather and grandmother separately instead of together as grandparents. But the result would be long-winded if both paternal and maternal grandparents are present.
I have tried in vain to get the phrase generator to process a more complex conditional like 'A and not B' in order to handle this situation. I have tried {?0&!1}, {?0}{?!1} etc. but all fail. Is it possible for the phrase generator handle this, either now in a future release?