What should a wife do who suffers from the domination of her husband's family and his father's control over her life and home, and her husband's threats of divorce when she tries to express her displeasure, which has made her think about separation?

Your husband does not have the right to force you to serve his family, and you are not obliged to obey him in this matter except as a voluntary act of kindness and charity on your part. His contentment with his father's mistreatment of you, his command for you to obey him, and his threat of divorce constitute injustice and ill-treatment on his part. He is obligated to fear Allah and not oppress you. We advise you to discuss this with him gently, explaining that serving his family is not an obligation upon you, and that his filial piety towards his parents does not justify him oppressing you. You may also seek the mediation of some righteous individuals. If that does not help, then send an arbitrator from his family and an arbitrator from your family. If that still does not help, consult with the wise ones from your family, weigh the harm of divorce against the harm of remaining with him, and choose the lesser of the two harms, while seeking Allah's help and supplicating.