The following male relatives are ghair- mahram and it is compulsory to adopt Hijab from them:
1- Maternal aunt’s husband
2- Paternal aunt’s husband
3- Male cousins from paternal uncle
4- Male cousins from paternal aunt
5- Male cousins from maternal aunt
6- Male cousins from paternal aunt
7- Sister’s husband
8- All male relatives of the husband except father-in-law. One should take special caution in regards to brother-in-laws.
One woman asked Rasulullah Sallallahu alahi wasallam as to whether she should adopt the Hijab from her brother-in-law. Rasulullah )صل الله علی( replied, “The brother-in-law is death.” Meaning just as death puts an end to life similarly by not adopting Hijab from the brother-in-law will put an end to one’s Deen. Therefore, one should fear the brother-in-law just as one fears death. This is because there is more Fitnah involved and that is why Rasulullah Sallallahu alahi wasallam emphasised this more and warned us in regards to it.
آج کل پردہ دری کا یہ نتیجہ نکلا جس کو سمجھے تھے کہ بیٹا ہے بھتیجہ نکلا
This is the consequence of not adopting the Hijab, Who you thought was the son turned out to be the nephew!
Adopting Hijab does not mean keeping yourself locked behind close doors. Rather if the house is small then one should fully
wrap oneself properly with a large shawl, covering the face and the body, and continue to do the house work. But if there is no one else in the home then it is not proper to be alone with any ghair mahram male relative. Also it is not permissible to speak to ghair- mahrams without necessity. If there is a need to ask to bring groceries or other household needs, then one should ask with a heavy voice from behind a curtain or barrier.
(SOURCE: https://www.ka.org.za/content/shari-hijaab)