A woman’s brother-in-law is considered a non-mahram; in other words, he is regarded like a stranger in terms of the rules of modesty and covering.
The categories of people exempted from the rules of covering are mentioned in the following Qur’anic verse:
“And tell the believing women to lower their gaze and guard their private parts and not to display their adornment except that which is apparent of it, and to draw their headcovers over their chests and not to reveal their adornment except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, believing women, those whom their right hands possess, male attendants lacking desire, or children who are not yet aware of women’s private aspects.” (Surah An-Nur, 24:31)
The brother-in-law is not included among these categories.
In family settings, depending on local customs and traditions, it may occur that people gather together or share meals. This may be tolerated to some extent. However, it remains necessary to observe the boundaries of Islamic modesty in all circumstances.