At 9 years old, I would say she doesn't have a choice. Does husband have full custody of her? If so, it's already been decided. Maybe you can reassure her that when (not if) you all move together into the new house, that she will have lots of opportunities to see her mom and can even stay for longer lengths in the summer. Maybe you guys can let her know she IS going with you, in a loving way, and talk about the excitement of her having a say in how her room will be decorated or things like that?

Best of luck. Keep us posted.