If she regularly dose niqaab and believes in the view that it is obligatory then she has to observe niqaab even in front of her male in-laws as well. It is one of the greatest doors of Satan to regulate such mixing and to make it normal.

I WANNA ask that i live in a joint family and my wife always wear hijab infornt of my brother and in fornt of my relative; so will she be sinful if she doesn’t wear niqab infront of them ? but she wears proper hijab with jalabiya…!!!!!