Hannelore...first, let me say how sorry I am to hear your family is going through all this. Sounds like mommie dearest doesn't want to be a mom nor a wife. How is your son at being a full-time father? Can your DIL take her daughter and fly to the states and stay without your son's approval? Is there a possiblity that could happen?
If they do divorce, I hope your son will take his daughter and raise her as best he can with you and your husband's help.
The way your DIL is behaving...she'll regret it one day when she realizes she's thrown away the most precious moments of her life with her child. She can never get that back. I hope your son realizes that the most important thing in his life is his child and will do everything to protect her and make a life for her regardless of how things turn out between him and your DIL. God bless and you all are in my prayers.