Does his wife become haraam for him if he commits zina with her sister?
Question

                If a man commits zina with his wife’s sister, does his wife become haraam for him?.
Praise be to Allah.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.