Allaah has forbidden a man to be married to two sisters at the same time. Allaah says, listing the women who are forbidden for marriage (interpretation of the meaning): 

“…and two sisters in wedlock at the same time, …”

[al-Nisa’ 4:23]

The scholars differed concerning one who commits zina with his wife’s sister – is zina like marriage to her or not? 

Ibn Qudaamah said in al-Mughni: If he commits zina with his wife’s sister, Ahmad said: He should refrain from having intercourse with his wife until three menstrual cycles have passed. 

It may be that her sister does not become haraam as the result of that, and neither do the other four, because this is not a marriage, and simply having intercourse does not prevent marriage (to her relatives), on the basis that intercourse with a slave woman does not prevent marriage to any the four. End quote. 

Those who regarded it as meaning that he cannot have intercourse with his wife until the woman with whom he committed zina has completed her ‘iddah quoted two hadeeths as evidence: 

1 – The report that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever believes in Allaah and the Last Day, let him not deposit his sperm in the wombs of two sisters at the same time.” 

2 – The report that the Prophet (peace and blessings of Allaah be upon him) said: “Cursed is the one who deposits his sperm in the wombs of two sisters at the same time.” 

But there is no basis for these two hadeeths, so they are fabricated. 

Al-Haafiz Ibn Hajar (may Allaah have mercy on him) said: 

There is no basis for either version. 

Ibn al-Jawzi mentioned the second version, but he did not attribute it to any of the books of hadeeth. Ibn ‘Abd al-Haadi said: I did not find any isnaad for it after searching for it in many books. End quote. 

Al-Talkhees al-Habeer (3/343). 

The correct view is that his zina does not make his wife haraam for him, but the 'iddah is obligatory if she (the other woman) wants to get married after she repents. Her ‘iddah does not have anything to do with his having intercourse with his wife. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said: 

It says in al-Rawd al-Murbi’ by Shaykh Mansoor al-Bahooti al-Hanbali: The one who has intercourse with his wife’s sister on the basis of a dubious marriage contract or blatant zina, his wife become haraam for him, until the woman with whom he had intercourse completes her ‘iddah. 

This means: if a man has intercourse with his wife’s sister – Allaah forbid – we say to him: Your wife is haraam for you until the woman with whom you had intercourse completes the ‘iddah that is required of a woman who commits zina. And if it so happens that the woman with whom he had intercourse becomes pregnant as a result, his wife is not permissible for him until the one with whom he had intercourse gives birth, even if the pregnancy lasts for four years! But we prefer the first view, which is that zina does not have any effect, because adultery cannot be regarded in the same terms as a valid marriage. End quote. 

Al-Sharh al-Mumti’ (12/133, 134) 

And Allaah knows best.