Is it acceptable in religion for a husband to treat his brother's wife in such ways, and does it permit him to insult his own wife for that reason?

It is not permissible for a woman to appear without hijab in front of her husband's brother, as this is an evil deed and a sin that requires repentance. Similarly, it is not permissible for a man to look at his brother's wife when she is without hijab; rather, he must lower his gaze. It is also not permissible for him to joke or laugh with her.

The Prophet (peace be upon him) warned against entering upon non-mahram women, and specifically highlighted the relatives of the husband, such as his brother and uncle. In the Hadith: "Beware of entering upon women." A man from the Ansar said, "O Messenger of Allah, what about the Hamw (in-law)?" He replied, "The Hamw is death." [Agreed upon]. The "Hamw" here refers to the husband's relatives other than his fathers and sons, such as his brother, nephew, uncle, and their sons, and so on. The danger lies in the ease of access to the woman and being alone with her without being questioned, unlike with a non-relative.

Everyone must fear Allah. A woman must observe hijab in front of her husband's brothers. The wife should advise her husband gently and explain the Islamic ruling, as the self is prone to evil, and one should not trust it, as Prophet Yusuf (peace be upon him) said: "And I do not absolve myself. Indeed, the self is a persistent enjoiner of evil, except for what my Lord has mercy upon. Indeed, my Lord is Forgiving and Merciful." [Yusuf: 53].

The Prophet (peace be upon him) warned: "I have not left behind me any trial more harmful to men than women." [Agreed upon].