Question

Is your wife’s sister your Mahram (non-marriageable relative)? Is it necessary for her to dawn the Hijab in front of you? Some people say that since you cannot marry your wife’s sister while you wife is alive, she is considered as Mahram while your wife is alive, and will become your non-Mahram after your wife passes away, therefore, you should maintain ties with her (i.e. wife’s sister) the way you would with other mahram women while your wife is alive.

Answer 1

Mahaarim (plural of Mahram) are those women who are permanently unmarriageable as a result of marriage. Since this is not the case of the wife’s sister, she is not considered as Mahram to her sister’s husband, and she must dawn the Hijab in his presence.
And Allah knows best.