I'd not be confident that you can leave it up to the warring parties to solve this on their own. It sounds like feelings are too raw.
Therefore, I'd take a picture of your cousin and his children, and put that in the book. Then a picture of the cousin and his wife, and give that to them. Then a picture of the children, with or without their mother, as she chooses, and give that to their mother. But I like taking pictures.