Ruling on looking at one's brother's wife and being alone with her

Question:
Is my brother's wife a mahram (a relative one may not marry) to me? I note that she does not observe hijab in my presence. Do I bear any sin?

Answer:
This has been mentioned before. Your brother's wife is not a mahram to you, nor is your uncle's wife, nor your maternal uncle's wife, nor your cousin's wife. They are not mahrams, and they must observe hijab in your presence. You must not be alone with any of them, because they are non-mahrams to you; they are not mahrams.
As for your father's wife, she is a mahram to you. Your grandfather's wife is a mahram to you. Your son's wife, your grandson's wife, and your daughter's son's wife are mahrams to you.
As for your brother's wife, your uncle's wife, your maternal uncle's wife, and your cousin's wife, no, they are not mahrams. It is not permissible for you to look at them or be alone with them. It is obligatory for them to observe hijab and not to uncover themselves before their husband's brother, nor their sister's husband, nor their husband's maternal uncle, nor his paternal uncle, nor his cousin.
Many women are very excessive in this matter, in many countries and in many places, and this is a great error. We ask Allah to guide them and to grant them uprightness and proper hijab.