A wife should not go out in front of the brother-in-law or be alone with him in the house, because the Prophet (saw) said:
“Beware of entering upon women.“
One of the Sahaabah said to him, “O Messenger of Allaah, what about the brother-in-law?“
He said: “The brother-in-law is death!“
[Bukhaaree, Fath al-Baari, 9/330]
Commentry By An-Nawawee:
An-Nawawee, may Allaah have mercy on him, said:
This hadeeth refers to all the relatives of the husband apart from his father and sons, who are mahrams for the wife and she is allowed to be alone with them; they are not described as “death.” It refers to the brother, nephew, uncle, cousin and other relatives of the husband whom she would be permitted to marry if she were not already married.
Because people customarily treat this matter so lightly, and a man may sit alone with his brother’s wife, the brother-in-law is likened to death, and he is the foremost among non-mahram men who should be prevented from doing so.