Is it permissible to lie to children about their aunt's bad reputation – she has a history of incest – to prevent them from interacting with her, out of fear for their morals, knowing that they were originally cut off from her?

The Islamic Sharia has prescribed the Hadd (prescribed punishment) for adultery and slander to protect people's honor. What was said about the wife's sister is one of two things: Either it is not legally proven, in which case it is considered slander, necessitating repentance and a refutation of the statement in front of the children, as Allah Almighty says: "And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and never accept from them any testimony, and those are the defiantly disobedient" (An-Nur/4). Or it is proven, and this would be by the testimony of four men who witness penetration, or by the adulteress confessing to herself. In this case, incest is a greater sin and crime than adultery with non-mahrams. Some scholars have said that the incestuous adulterer should be killed absolutely, while the majority of jurists believe that his punishment is the same as that of an adulterer with a non-mahram. If the act is proven and she does not repent, or the Hadd is not carried out on her (if it is lashing), it is permissible to warn the children and others who might be influenced by her. This falls under permissible backbiting, provided that the warning is for the purpose of sincere advice and not out of personal motivation.