What is the ruling on marrying the sister of my wife whom I divorced?
It is prohibited for you to marry the sister of your divorced wife before the elapse of her `Iddah (woman's prescribed waiting period after divorce or widowhood). This will be the case whether your Talaq (divorce pronounced by a husband) is irrevocable by Khul` (divorce at the request of the wife in return for compensation to the husband), by a thrice divorce or revocable. When her `Iddah elapses whether by menstruation, if she is one of those who menstruate, by giving birth if she was divorced during pregnancy, by the elapse of three months from marriage if she is in menopause or too young to menstruate, in this case, it will be permissible to marry her sister. It is prohibited to combine between your wife and her sister in marriage while your wife is still in her `Iddah. (Part No. 18; Page No. 237) Allah says when He mentions the women unlawful to be married,
...and two sisters in wedlock at the same time, except for what has already passed; verily, Allâh is Oft-Forgiving, Most Merciful.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.