Marrying the Sister during the Ex -Wife's Iddah

Question: Because my wife has not had children for the past ten years due to her illness, arrangements have been made with her consent for me to marry her younger sister. When it came to our notice that a sister cannot be married when her sister is married to the same husband, I divorced my wife and then married her sister before the Iddah was over. Now my first wife wants to remarry me. What am I to do?

Answer: It is not permissible and Haraam to marry a woman's sister. aunt or niece when she is still married to one or still competing her lddah after divorce. The Qur'aan clearly states that two sisters may not be joined in one marriage and a Hadith also makes it clear that a person may not deposit his waters into the wombs of two sisters.

When Hadhrat Daylami RAH. became a Muslim, he was married to two sisters. Rasulullaah SAW. instructed him to choose one of the two and divorce the other.
Your marriage to your wife's sister is invalid since your wife was still passing her Iddah. You should therefore separate from her immediately and marry her only once the Iddah has expired. Since you had divorced your wife thrice, you may not remarry her until Halaalah has been done. However, even after the Halaalah, you may not remarry her if her sister is still married to you. People should make an effort to stop the practice of marrying two sisters, which is a grave sin and which attracts the anger of Allaah. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.3