What is the ruling on a two-month marriage that resulted in a child, between a man who divorced his wife to marry her pregnant sister, then returned to his first wife? Is this marriage permissible, and is what the husband did forbidden, and is this child legitimate?

This marriage is invalid by the consensus of Muslims, because the marriage of a woman observing her iddah due to divorce or death is invalid during her iddah, based on the verse: "And do not determine to consummate the marriage contract until the prescribed period reaches its end." The iddah of a pregnant woman ends with childbirth. If she became pregnant through unlawful sexual intercourse (zina), the matter is even more egregious, and the pregnancy cannot be attributed to this husband under any circumstances, based on the Prophet's (peace be upon him) saying: "It is not lawful for a man who believes in Allah and the Last Day to irrigate with his water the crop of another." All who participated or knew should repent to Allah and regret what they have done, and disavow attributing this child to other than its father.