This Hadith mentioned is reported in Sahih Al-Bukhari, vol. 7, Book of Nikah (Marriage), Hadith no. 159. It reads as follows:
`Uqbah Ibn `Aamir, may Allah be pleased with him, quotes Allah’s Messenger as saying, “Beware of entering upon women.” A man from the Ansar said, “O Messenger of Allah! What about Al-Hamu, or the wife’s in-law (the brother of her husband or his nephew, etc.)?” The Prophet, peace and blessings be upon him, replied: “The in-law of the wife is death itself.”
The Hadith is also reported in Sahih Muslim, Kitab As-Salam (Book on Salutations and Greetings), Chapter 8, Hadith no 5400.
Commenting on this Hadith, Imam An-Nawawi, may Allah bless his soul, states: “Al-Layth Ibn Sa`d holds that the “the in-law” refers to a relative of the husband other than his father and sons (who are Mahram (unmarriageable kin) to his wife), such as his brother, nephew, and cousin, etc., with whom marriage would be permissible for her, if she were to be divorced or widowed
As for his saying “the in-law is death”, it means that you are supposed to be very cautious of him as evil is most expected from him. This is because the in-law, contrary to the stranger, can easily approach the lady and violate her privacy, without people blaming him for doing so.
The in-law here stands for husband’s relatives other than his fathers and sons. As for the husband’s father and sons, they are Mahram to his wife and they are allowed to be alone with her. They don’t fall under the category of “the in-law is death”. Those who are described of death are the husband’s brother, cousin, uncle, and all those who are not Mahram for the wife. People usually take it easy and find no problem with the husband’s brother being in private with the former’s wife. This is “death” itself as the in-law is most worthy of the prohibition.
Al-Mazari is of the opinion that the in-law refers to the husband’s father, but this not correct and it is rejected.
Shedding more light on the Hadith, the prominent scholar Sheikh Yusuf Al-Qaradawi, says:
“The Prophet, peace and blessings be upon him, particularly warned women concerning Khulwah (being alone) with male-in-laws such as the husband’s brother or cousin, since people are quite negligent in this regard, sometimes with disastrous consequences. It is obvious that a relative has easier access than a stranger to a woman’s quarters, something concerning which no one would question him. The same is true of the wife’s non-Mahram relatives, and it is prohibited for any of them to be in Khulwah with her.
By saying “the in-law is death” the Prophet, peace and blessings be upon him, meant that there are inherent dangers and even destruction in such privacy: Religion is destroyed if they commit sin; the wife is ruined if her husband divorces her out of jealousy; and social relationships are torn apart if relatives become suspicious of each other.
The danger lies not merely in the possibility of sexual temptation. It is even greater in relation to the possibility gossip about what is private and personal between the husband and wife by those who cannot keep secrets to themselves and relish talking about others; such talk has ruined many marriages and destroyed many homes. In explaining the meaning of ““the in-law is death,” Ibn al-Atheer says, “It is an Arabic figure of speech like, ‘The lion is death’ or ‘The king is fire,’ which means that meeting a lion is similar to facing death and a confrontation with a king is like being in the fire. Thus, privacy between an in-law and a woman is far more dangerous than in the case of a stranger because he might persuade her to do things against her husband’s wishes, such as asking him for things he cannot afford, nagging him, and the like.”