I am a middle child with two sisters who don't get on. I have remained neutral and done what Anya suggests for 20 years. When my younger sister lost her husband, my older one (already a widow) came to the funeral and for a while after that they started seeing each other again, but it did not last, they are too different and end up hurting each other when old resentments start to come out. It is a much harder situation with your own son and grandson, I agree that you should stay out of it, be a good listener, let the matter rest, but if it comes up, and you get the chance, make mild comments that help them to see the other side's point of you. But you have to be very careful if you do this or they will both think you are on the other's side and you will lose their trust. Hopefully by the time you pass on they will have matured/things will have cooled and they will both attend and be civil.