The ruling on shaking hands with one's wife's sister and being alone with her?
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Question : May Allah reward you. Allah the Exalted said: [An-Nisa 4:23] The question is: Is it permissible to examine a wife's sister, look at her, shake her hand, and greet her?
Answer : No, it is not permissible for the husband to greet his wife's sister, nor is it permissible for her to show her face to him, because she is not one of his mahram (relatives one may not marry).
And every woman whom it is permissible for you to marry, it is not permissible for her to show her face to you. This is the rule: every woman whom it is permissible for you to marry, it is not permissible for her to show her face to you. Is it permissible to marry the wife's sister or not?
It is permissible, because Allah has not forbidden the wife's sister; rather, He has forbidden marrying both of them together. For this reason, He said, "Forbidden to you are your mothers..." up to His saying:[An-Nisa 4:23] Those who say that the wife's sister is forbidden to the husband are mistaken. We say she is not forbidden. What is forbidden? It is marrying both together, as Allah the Mighty and Majestic said: [An-Nisa 4:23] Therefore, it is not permissible for the wife's sister to show herself to the husband, nor to speak to him in a manner that is submissive in tone, nor for him to be alone with her, nor to joke with her, nor to shake her hand, because she is a non-mahram (foreign woman).
and that you take [in marriage] two sisters simultaneously, except for what has already occurred
وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَAnswer : No, it is not permissible for the husband to greet his wife's sister, nor is it permissible for her to show her face to him, because she is not one of his mahram (relatives one may not marry).
And every woman whom it is permissible for you to marry, it is not permissible for her to show her face to you. This is the rule: every woman whom it is permissible for you to marry, it is not permissible for her to show her face to you. Is it permissible to marry the wife's sister or not?
It is permissible, because Allah has not forbidden the wife's sister; rather, He has forbidden marrying both of them together. For this reason, He said, "Forbidden to you are your mothers..." up to His saying:
and that you take [in marriage] two sisters simultaneously
وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِand that you take [in marriage] two sisters simultaneously
وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، لقاء[18 من 41] حكم مصافحة أخت الزوجة والخلوة بها؟!! الشيخ ابن عثيمين - مشروع كبار العلماء — youtube.com
The Arabic text is a machine transcription of the audio recording.