The husband of a sister is not a mahram to the other sister

Question: Is the husband of a sister considered a mahram to the other sister temporarily, such that it is permissible for him to be alone with her? And what about the maternal aunt of the daughter?

Answer: No, the husband of a sister is not a mahram to her. The husband of a sister, the maternal aunt, and likewise the paternal aunt—her paternal aunt is not one whom it is forbidden for him to marry along with the daughter, so he does not combine them in marriage. The same applies to her maternal aunt. This is only a temporary prohibition, and the husband of her sister was not a mahram to her. Likewise, the husband of her paternal aunt, and the husband of her maternal aunt, and the paternal aunt herself, the paternal aunt of the wife, the sister of the wife, and the maternal aunt of the wife are not mahrams to the husband mentioned. This is because they are prohibited to him. Whenever he divorces their sister, or their brother's daughter, or their sister's daughter, it is permissible for him to marry after the waiting period. The result is that some people are lenient with some women, such as the husband of their sister, and some people are lenient with the brother of their husband, or the paternal uncle of their husband. This is an error and is not permissible.
Therefore, it is not permissible for a woman to be lenient with the brother of her husband, for he is not a mahram to her. His paternal uncle is not a mahram to her, nor is his maternal uncle. The only mahrams are her father and her son.