Question Summary:
Will zina with my brother in law break my sisters’ marriage?

Question Detail:

If a woman has intimate relations with her sister's husband, does the nikaah break between the sister and the brother in law?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
 
As-salāmu ‘alaykumwa-rahmatullāhiwa-barakātuh.
 
If a man commits adultery with his sister in law, his marriage to his wife will remain intact. It should be remembered that adultery is a grave sin and the perpetrators of such a heinous sin should repent to Allah Ta’aala abundantly and seek His forgiveness.[1]
 
And Allah Ta’āla Knows Best
 
Saanwalibn Muhammad,
Student DarulIftaa
UK   
 
Checked and Concurred by,
Abdul Azīm bin AbdurRahman,
Ismail Desai.