“Say to the believing men that: they should cast down their glances and guard their private parts (by being chaste). This is better for them. Say to the believing women that: they should cast down their glances and guard their private parts (by being chaste)…”
[Qur’an- 24: 30 31,]
No one has the right to regard as permissible that which Allah has forbidden, or to regard as forbidden that which Allah has permitted. The women, as well as men must observe hijab before the non-mehram relatives (of opposite sex) the way they observe it before all other non-mehrams.
So is the case with sister in law and brother in law for the believing man and believing women. Prominent Islamic scholar, Dr. Israr Ahmed explains the fitnah that seeps in Muslim Ummah that does not regard them (spouse’s sibling of opposite sex) as non-mehram.