The Prophet (peace and blessings be upon him) strongly warned concerning the in-laws, since people sometimes mix freely with them without any limits which may cause severe problems in families.
In his explanation on this issue, Sheikh Ahmad Kutty, an Islamic scholar states that ‘a person’s wife is not allowed to treat her brother-in-law as a mahram; because he is not her mahram and, as such, he is no different from any other stranger for her as far as the rules of hijab and isolation are concerned.
A mahram is someone whom one is not allowed to marry at all in any circumstance. Someone’s wife is allowed to marry his brother, if, God forbid, he divorces her or dies. In this way, the bar for marriage that exists at present does not mean they are mahrams to each other as it is only a temporary bar.
Furthermore, the Prophet (peace and blessings be upon him) warned us against isolation between the sexes in similar relations: He said, “In-laws are akin to death!” In other words, they face a greater risk of falling to sins than strangers, because they have more chances to meet and interact, and the result would be far more disastrous than death. Therefore, let us guard against all the doors that lead to temptation, for as the Prophet (peace and blessings be upon him) said, “The devil runs through the blood veins of the children of Adam!” They can only protect themselves against his snares and temptations through remembrance of Allah and by complying with His orders.
In conclusion, a person’s wife must observe full hijab in front of her brother-in-law, and she is not allowed to be alone with him under any circumstances.’