Your ex is the only one who can do anything, he has to make a will and have the girls named in it and state what it is he is leaving to them. There is not legal right to being anyone's next of kin but his wife at the time of his death would have to be in his will to be able to inherit anything from him.
You should encourage him to have the idea himself though, men are stubborn asd you no doubt know, you could do this by making up a story about a friend whose children were left with nothing because their dad did not think to make things legal in the form of a will,