All Perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as that of his family and all his companions.
Dear brother, you must know that combining between two sisters as wives is absolutely forbidden and it is a fundamental in Islamic legislation which is proved by Quran, Sunnah and the consensus of Muslim scholars. Allaah Says (that means): {… and that you take [in marriage] two sisters simultaneously…}[Quran 4:23].
This prohibition is eternal till the Day of Judgment and covers all people; it is not peculiar to someone or certain cases. So, you are not allowed to marry the sister of your wife as long as your wife is in your wedlock.
We would like to say to you clearly that it is obligatory upon you to ask your sister-in-law to leave your house to live with her brother. You are not her Mahram (i.e. a spouse or a person whom one is permanently prohibited from marrying due to suckling, marital, or blood relations) and it is forbidden for you to be in seclusion with her or to shake hands with her. It is also forbidden for her to appear in front of you without observing complete Hijab. We fear that you will become attached to her passionately, if it has not already happened, and it may lead to evil consequences.
Therefore, you should fear Allaah and should not think of combining between two sisters as wives which is absolutely forbidden. She has to leave your house to live with her brother or any other Mahram, as this is safer and far away from any seduction.
Allaah Knows best.