What is the ruling regarding a married man who has sexual relations with his wife's sister?

A wife must satisfy her husband's need for intimacy to the best of her ability. If she refuses without a legitimate Shar'i reason, that is a great sin, for the Prophet, peace and blessings be upon him, said: "If a man calls his wife to his bed and she refuses to come, and he spends the night angry with her, the angels curse her until the morning," and he also said: "If a man calls his wife for his need, she should come to him, even if she is at the oven." As for having intercourse with the wife's sister, that is a heinous crime. If a married person commits adultery, they deserve to be stoned to death, and punishments are to be carried out by the ruling authority, not by individuals.