The Question of Mixing With the Non Mahram or Who Are Only Temporarily Her Mahrams
In the name of Allāh, the Most Gracious, the Most Merciful;
All the praise and Thanks is due to Allāh, the Lord of al-`ālameen. I testify that there is none worthy of worship except Allāh, and that Muhammad Sallallāhu `alayhi wasallam is His Messenger
The Messenger (Sallallāhu`alayhi wasallam) warned against having too relaxed an attitude towards the matter of women mixing with men who are not her mahrams, or who are only temporarily her mahrams, like her sister’s husband or her husband’s brother , nephews and so on. Prophet Muhammad (Sallallahu`alayhi wasallam) said: "The brother of a woman's husband is death.''
Imam Al-Bukhari recorded that `Uqbah bin `Amir (radiyallahu`anhu) narrated, who said:
The Messenger of Allah (Sallallahu`alayhi wasallam) said: "Avoid (entering a place) in which are women (uncovered or simply to mix with them in seclusion)"
A sahabi from the Ansar then asked: “Allah's Messenger (Sallallahu`alayhi wasallam), what about Al-Hamu the in-laws of the wife (the brothers of her husband or his nephews etc.)?”
The Prophet (Sallallahu`alayhi wasallam) replied: "The in-laws of the wife are death itself."
[Sahih Bukhari, Book 62, Hadith No: 159]
The hadith indicates many people are not concerned the matter of women mixing with men who are not her mahrams, or who are only temporarily her mahrams, like her sister’s husband or her husband’s brother, nephews and so on.
The Prophet (Sallallāhu`alayhi wasallam) reminded: "The in-laws of the wife are death itself." which means the brother-in-law entering upon women place might expose to some untoward incident as a result. They might never think it would happen because they trusted their brother-in-law! This in reality is negligence, because no man is alone with a non-mahram woman but the Shaytān is the third who is present with them, and the situation is exposed to temptation, except for those on whom Allāh has mercy.
[Masā’il wa Rasā’il, Muhammad al-Mahmud al-Najdi, p. 11]
And Allāh Almighty Knows best.