I learnt that a friend of mine is committing adultery with his sister-in-law. What should I do, knowing that I am certain about it?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

First of all, you have to do your best to advise this person. You must remind him of Allaah and clarify that what he is doing is horrible. You have to inform him that committing adultery with one’s sister-in-law is one of the gravest major sins and one of the most abominable crimes. For further information see Fataawa 84637 and 85425.

Your words may cause him to repent and give up such grave sins. However, if he insists on his wrongdoing and does not listen to you, then you may threaten, in a serious tone, to tell his brother. This may be an effective method; nonetheless if he insists on not listening to you despite your threat, then you have to advise his brother and warn him, in general, against allowing his male kin to be with his wife.

Beware of direct accusations because accusing someone of committing adultery requires four witnesses, or else the accuser will subject himself to the corporal punishment prescribed in the Sharee’ah, if he is in a place where it is applied.