Should a wife have to choose between caring for her granddaughter, the daughter of her divorced son, who is Christian and does not receive adequate care from her mother, and the satisfaction of her husband (the child's grandfather), who rejects the presence of children?

The person most entitled to custody of a child is the mother. The mother's right is forfeited if she is immoral or dissolute, which applies to the mother of the aforementioned child. If the mother is absent or has remarried, the maternal grandmother is most entitled to custody. However, if she is a disbeliever, the right transfers to the paternal grandmother. Since custody is a religious obligation, and there is no one left to care for the child, you must assume her custody. It is not permissible for you to obey your husband in abandoning her, because there is no obedience to a creature in disobedience to the Creator. You should convince him of the Islamic ruling. If he insists on his opinion, then you must assume her custody, even if it leads to separation.