A brother-in-law is not a Mahram to his wife's sister
The Qur'an mentions that it is Haram (prohibited) for a man to marry two sisters at once, does this mean that a brother-in-law is considered a Mahram (unmarriageable relative) to his wife's sister?
A brother-in-law is not considered a Mahram for his wife's sister because it is permissible for him to marry her if his marriage to her sister is terminated either by the death of the latter or by divorce. Marrying two sisters at once is Haram lest hatred, enmity, and severing ties of kinship should arise among relatives. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.