A married man is in love with his brother’s wife and has physical relations with her, but without having sexual intercourse.
What is the ruling of Islam on this issue, knowing that we live in a country that does not adopt Sharee‘ah (Islamic law)? What is the ruling if this happened in a Muslim state that applies the Sharee‘ah law? Is this considered a major sin and how should the guilty parties repent?
May Allaah reward you.

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.

‘Uqbah ibn ‘Aamir reported that the Prophet, sallallaahu ‘alayhi wa sallam, said, “Beware of entering upon women.”  A man from the Ansaar asked, "What about [male] in-laws?”  The Prophet, sallallaahu ‘alayhi wa sallam, replied: "[Male] in-laws are death.” [Al-Bukhari and Muslim]

People are usually negligent regarding this issue and allow the brother to meet the wife of his brother or the wife of his nephew privately, forgetting that the husband’s male relatives should be prevented from doing so, even more than strangers.

The Prophet, sallallaahu ‘alayhi wa sallam, likened the husband’s male relatives to death for more than one reason:

- A private meeting between the husband’s wife and his male relative may destroy their religion if they commit a prohibition.

- If they commit a sin, it will prove fatal because in an Islamic state this crime is punishable by stoning to death.

- This can also be fatal if the husband begins to suspect that his wife has illicit relations with someone else; he might kill her or divorce her because of his jealousy.

Hence, the meaning is “Beware of private meetings between the husband’s male relatives and his wife as much as you fear death.”

In conclusion, this is one of the most dangerous causes of Fitnah, (trial) because evil is expected more from the husband’s male relatives than others. This is because they can get to the woman and meet her privately without any objection, especially in families who do not know this ruling.

In any case, Islam prohibits such shameful acts and this man has betrayed his brother, and this woman has betrayed her husband. Both of them should imagine what would happen if the brother discovers their betrayal and dishonesty and the shame that they would feel if Allaah The Exalted exposes them in front of all people on the Day of Judgment.

This man and woman must repent to Allaah The Almighty and never tell anyone about this. They must also block the way before the devil by refraining from being alone with each other,   speaking on the phone to each other and so on.

Allaah Knows best.