What should a person do with his father who treats his wife badly, while the wife serves the father and brothers, and the father holds her accountable for every little thing?

You must advise your father gently and kindly, and explain to him that his stubbornness with your wife and his treatment of her in that manner are impermissible. Allah has commanded kindness to all people, and it is even more incumbent to be kind to relatives and in-laws. If your wife is voluntarily serving your father and brothers, this emphasizes the necessity of being kind to her, as Allah Almighty says: "Is the reward for good [anything] but good?" Even if she falls short in her service, she should not be blamed; for the volunteer who does good is not to be blamed, as Allah Almighty says: "There is no blame upon the doers of good."