You can't change them but you can refuse to participate in their ridiculous war. Stop letting them draw you into the drama by refusing to discuss one with the other. Walk away, don't listen and determinedly ignore their nonsense. Help your daughter do the same perhaps by making a joke of Dad and Nana's silly feud and laughing together about their childish behavior. Treat them both just like the brats they are and show your daughter the example of real grownup behavior by being reasonable, courteous and kind in your dealings with your family and others even when they don't return the favor. Maybe your mum and husband will be able to change a little if you succeed in showing them how decent people behave even if they are forced to be in contact with family members who don't behave decently. It's worth a try and it may even give you some relief if you are able to distance yourself emotionally from their ugly emotions. Good luck.