Some often ignored things in our societies nowadays.
It is impermissible to see non-Mahram male without any valid reason
Nowadays, men’s looking at women is considered wrong, but women don’t consider seeing men and meeting them as a sin although there is a danger of the doors of immodesty being opened with seeing non-Mahram males without a reason accepted by the Shariah. The Sahabah would consider this very wrong, as is reported about Hadhrat Ma’adh Bin Jabal (May Allah be well pleased with him):
Translation: When he came to his house, he saw his wife peeping out of a hole in the tent, as a reprimand, he slightly tapped her. (Al Tabaqat Ul Kubra Li Ibn Sa’ad, Ma’adh Bin Jabal)
Shaking hands with non-Mahram men
Nowadays, following the fashion and western culture is at its peak. Especially, those who live in mixed cultures are very much affected by this. That’s why mixing of men and woman, eating at the same table, shaking hands with each other and contacting each other is not considered wrong at all. Instead, those who consider this as a sin are termed uneducated and with old fashioned.
When looking at a non-Mahram itself is a major sin, then imagine how worse is a sin is to touch her and shake hands with her, as by touching there is a great danger of corruption.
Touching non-Mahram women is such a grave sin that the punishment given to the one who does it is much more harsh than the agony felt on the head being pricked by an iron needle. As given in the Hadith:
Translation of Hadith: It has been narrated on the authority of Hadhrat Ma’qil bin Yassaar (May Allah Ta'ala be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: It is better that your head be pricked with an iron needle than that anyone from among you touch a non-Mahram woman. (Mo'jam Kabeer, Hadith No. 16880; Kanz Ul Ummal, Hadith No. 13065)
When non-Mahram man and woman meet in seclusion, then the third one among them is Satan who encourages them to sin and presents the sin in a beautiful form. It is narrated on the authority of Hadhrat Abu Umama (May Allah be well pleased with him):
Translation of Hadith: The Holy Prophet (Sallallahu alaihi wa sallam) said: You avoid being alone with women. By Him who holds my life in His hand, no man is alone with a non-Mahram woman but that Satan comes between them.
Purdah with brother-in-law
Islam has always kept aims and objectives in mind. Because the intent is to protect the chastity and honor of women dignity, checks have been imposed on every quarter from where there is a danger to the dignity and chastity of women even though it may be in the home itself. A brother-in-law who is the brother of husband may turn out to be dangerous to the chastity of the chastity of the lady. That's why the Holy Prophet (Sallallahu alaihi wa sallam) had declared him as death for sister-in-law. This is because for a modest and chaste lady, any attack on her chastity is death itself.
Hence, there is a Hadith in Sahih Bukhari and Sahih Muslim:
Translation of Hadith: It has been narrated on the authority of Hadhrat ‘Uqba bin Aamir (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: Avoid visiting women. A Sahabi asked: O Prophet of Allah (Sallallahu alaihi wa sallam), what do you say about the husband’s brother? The Holy Prophet (Sallallahu alaihi wa sallam) said: The husband’s brother is death. (Sahih Bukhari, Hadith No. 4831)
The meaning of this is that the way everybody wants to save oneself from death and if there is a life-threatening situation, then everybody would prefer saving one’s life, in the same way in spite of there being a relationship, one should prefer practicing Purdah with the husband’s brother as well.