Adultery with Sister-in-Law

In the name of Allah, Most Compassionate, Most Merciful,

As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.

Indeed, committing adultery is one of the major sins in Islam and a despicable act in the eyes of Allah Ta’alā. Allah Ta’alā says:
وَلَا تَقْرَبُوا الزِّنَا إِنَّهُ كَانَ فَاحِشَةً وَسَاءَ سَبِيلًا (سورة الاسراء: 32)
Translation: “And do not go close to adultery. Indeed, it is ever an immorality and it is an evil way.”
(Surah Al-Isra:32)
Furthermore, Rasulullah (Sallallahu Alaihi Wasallam) advised one to be very cautious around one’s in-laws. See the following Hadith:
عَنْ عُقْبَةَ بْنِ عَامِرٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏”‏ إِيَّاكُمْ وَالدُّخُولَ عَلَى النِّسَاءِ ‏”‏ ‏.‏ فَقَالَ رَجُلٌ مِنَ الأَنْصَارِ يَا رَسُولَ اللَّهِ أَفَرَأَيْتَ الْحَمْوَ قَالَ ‏”‏ الْحَمْوُ الْمَوْتُ ‏” (صحيح مسلم: 2172)
‏Translation: Uqba bin Amir reported Allah’s Messenger (Sallallahu Alaihi Wasallam) as saying: Beware of getting, into the houses and meeting women (in seclusion). A person from the Ansar said: Allah’s Messenger, what about husband’s brother, whereupon he (Sallallahu Alaihi Wasallam) said: Husband’s brother is like death. ‏‏(Saheeh Muslim: 2172)
Nevertheless, the rulings pertaining to committing adultery with one’s sister in law are the same as committing adultery with a common woman. In an Islamic country, an unmarried individual found guilty of adultery would receive 100 lashes and a married individual found guilty of adultery would be punished by stoning to death.[i]

[i] الزَّانِيَةُ وَالزَّانِي فَاجْلِدُوا كُلَّ وَاحِدٍ مِّنْهُمَا مِائَةَ جَلْدَةٍ ۖ وَلَا تَأْخُذْكُم بِهِمَا رَأْفَةٌ فِي دِينِ اللَّهِ إِن كُنتُمْ تُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ ۖ وَلْيَشْهَدْ عَذَابَهُمَا طَائِفَةٌ مِّنَ الْمُؤْمِنِينَ – 24:2