What is the correct course of action to take in dealing with a sister's husband who assaulted his eldest daughter and got her pregnant, keeping in mind the desire for concealment and avoiding scandal, and knowing that this is the second time he has assaulted her?

Incest is the most dangerous type of zina (unlawful sexual intercourse), and there is no sin upon you for concealing it the first time for his repentance and regret. However, the daughter should not have allowed him to be alone with her if she sensed any suspicion from him. You are not obliged to report the matter to the authorities; indeed, concealment is preferable. Nevertheless, measures must be taken to protect the daughter from him in the future, and scholars should be consulted. A lawful way can be pursued to deliver the daughter's pregnancy without exposing her secret, and abortion of the fetus is forbidden, especially after the soul has been breathed into it. It is not permissible for the father to have custody of his daughters, and efforts should be made to marry off those of them who have reached marriageable age.