HI, That is very difficult. If your son decides to move, he needs to have an honest and direct conversation with his daughter. Addressing her feelings, and making a plan on how to help her continue to feel bonded with them.
Children can become resilient with caring words and attention. He can provide this long distance for a while, and hopefully she will feel positive about their relationship.
As for the Mom, I cannot speak for her, but would hope she may consider some alternate living options for the benefit of all the children, who will be losing siblings. Gina