The ruling on a woman being alone with her sister’s husband while traveling and shaking his hand?

Question:
May Allah reward you with good and do you well. Our sister is also asking about the ruling on being alone with her sister’s husband while traveling and shaking his hand. She asks for your guidance and clarification of the evidence in this matter, because she notices a great deal of laxity regarding this issue. May Allah reward you with good.

Answer:
A sister’s husband is a non-mahram. He is a non-mahram to his wife’s sister. It is forbidden for her to shake his hand, and it is forbidden for her to be alone with him, because he is a non-mahram like any other man who is not among her mahrams. The fact that he is her sister’s husband does not make him a mahram or remove his status as a non-mahram. Therefore, it is forbidden for her to be alone with him, it is forbidden for her to shake his hand, and it is forbidden for her to travel with him without a mahram, because he is a non-mahram in every sense of the word.
This is a matter that many people are ignorant of, because their custom has been to consider a sister’s husband a mahram to his wife’s sister. This is a clear error. They see laxity in this, and they think she may uncover her face in his presence, and that she may shake his hand, and that he may greet her with a handshake or a kiss, just as he greets his mahrams. All of this is forbidden and not permissible. It involves fitnah (trial, discord or temptation) and evil. The only ones who are mahrams to her are her mother’s husband or her daughter’s husband, or her husband’s father. These are the ones who are mahrams to her. Yes.