Answer:
It is not permissible for a man to get married to a woman and her sister at the same time. This is clearly mentioned in the Holy Quran (Surah 4, verse 23).
From your query it seems that you have not been following the Islamic teaching regarding doing pardah with your sister in-law. Islam teaches that one is not to intermingle, interact and unnecessarily talk with one’s sister in-law. Not following this teaching of Islam leads to many problems, like the problem that you and your sister in-law are in.
However, it is never too late. Do sincere tauba (repentance). Ask for Allah’s forgiveness and do not interact and talk with your sister in-law again. Also, increase your knowledge of Islam, stay away from all sources of evil and spend your time with pious and practicing Muslims.
Mufti Faizal Riza
7/9/2011