Will the granddaughter be sinful if she supports her grandfather in divorcing his wife, who has caused problems and threats to him and his children, especially since this might improve his strained relationship with his children?

It is permissible to advise the father to separate from his wife if his remaining with her causes him harm. The advice should be given with wisdom and a gentle approach. Children must honor their father, treat him kindly, and not abandon him or mistreat him. They should look for a suitable wife for him if he needs to marry. If he does not accept the advice, then they must continue to be gentle and dutiful towards him.