Is it permissible to uncover one's head (hijab) in front of one's sister's husband due to his frequent presence in the home?

It is not permissible for a Muslim woman to remove her hijab (definition) or display her adornment to her sister's husband, because this prohibition is temporary and ceases upon the termination of the marital relationship. His frequent presence in the house does not permit this; rather, it necessitates more covering. Allah Almighty says: (And tell the believing women to lower their gaze and guard their private parts and not display their adornment except for what is apparent thereof. And let them draw their headcovers over their bosches and not display their adornment except to their husbands or their fathers...) to the end of the verse. Furthermore, one must beware of being alone with a sister's husband or a husband's brother, due to the Prophet's (peace be upon him) saying: "Beware of entering upon women." A man from the Ansar said, "O Messenger of Allah, what about the in-law?" He said, "The in-law is death," and due to his saying: "No man should be alone with a woman except with a mahram (definition)."