Kindly refer to the following link(s):
Hijab in front of brother in law in a Joint Family System | By Assim Al Hakeem
The Muslim woman and her brother-in-law | Assim Al Hakeem
Non-Mahrams free mixing with made up relationships! | Assim Al Hakeem
Mixing between cousins of opposite genders & Islam promoting segregation
Why does Islam promote segregation among opposite genders?