Is it permissible for a woman to uncover her head in the presence of her sister's husband? Is my brother-in-law mahram to me? Can I uncover my head in his presence?

Dear Brother / Sister,

Your sister's husband is your brother-in-law and you are her sister-in-law. There is a temporary mahram state between him and you in terms of marriage. That is, a man cannot be married to two sisters simultaneously; therefore, there is a mahram state between them because marriage between them is not permissible. Apart from that, a man cannot travel alone with his wife's sister and he cannot be alone with her in a closed place or in an open place where nobody can see them.

In addition, it is not permissible for them to shake hands and to kiss each other's hands. As for their speaking, it is something different. It is permissible for them to speak to each other if they act in a chaste way and carefully, without causing any mischief and sins. Besides, they can come together within the family, visit each other and sit in the same place.

Accordingly, you need to cover all of your body except your face and hands when he is present. We learn the criterion about it from the Prophet (pbuh).

According to what Hz. Aisha narrates, once, Asma, her sister, entered into the presence of Messenger of Allah wearing a thin dress that showed her skin. The Prophet (pbuh) turned his face away and said to his sister-in-law:

“O Asma! When a woman reaches the age of puberty, it is not permissible for her to display her parts of body except this and this.” And, he pointed to his face and hands. (Abu Dawud, Libas: 32)

The same decree is valid for the husband of your mother's sister and your father's sister. They are like the husbands of your sisters. The same limitations as the ones that are valid with your sister's husbands are also valid with them. That is, as long as you live and unless you divorce, your husband cannot marry your maternal and paternal aunts and their husbands cannot marry you. 

In addition, the husbands of your nieces and the husbands of your husband's nieces are non-mahram to you. There exists a permanent, not temporary, mahram state between you and them, unlike your brothers-in-law

(Mehmed Paksu, Aileye Özel Fetvalar, Nesil Yayınları, 3. Baskı, Mart 1999, p. 158-159)

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