What is the ruling of the Sharia on a married man having a love affair with his brother's wife, which includes acts of love without adultery, and are these acts considered major sins, and how can one repent from them, taking into account the difference in ruling between it being in an Islamic or non-Islamic country?

The meaning of 'Al-Hamu' in the Hadith refers to the husband's relatives with whom she would be permissible to marry if she were not already married. It has become a common custom for people to be lenient in this matter, where a brother might be alone with his brother's wife, or a nephew (brother's son) or a niece (sister's son), even though this is more prohibited than being alone with a strange man. The Prophet's (peace and blessings be upon him) likening of 'Al-Hamu' to death indicates that being alone with 'Al-Hamu' could lead to the destruction of one's religion if a sin occurs, or to death if a sin occurs and the prescribed punishment is applied, which might be stoning to death, or to the separation of the wife if the husband's jealousy leads him to divorce her. The summary of the ruling is that this is one of the widest and most dangerous doors to temptation, and evil from 'Al-Hamu' is more expected than from others. Therefore, the ruling of Islam in this matter is a strict prohibition. Both the man and the woman must repent, conceal what happened, and avoid being alone together and conversing on the phone or otherwise.