I have 2 friends who are brothers and the wife of the younger brother in the past committed zina with the older brother. Both brothers are aware of this but the younger brother is demanding that the older brother have a normal sister in law/brother in law relationship with his wife ie. go to there house for dinner and play happy familes and treat her as he would a sister in law he didnt have a history with and if the older brother doesnt do this then he will break ties with him but the older brother is saying that it would be better if he kept his distance as it would be wiser and safer for everyone involved as there is no need for him to have any sort of relationship with his new sister in law except to acknowledge that she is now his sister in law and thats it.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The wife of one’s brother is not among his Mahram women, so he should deal with her on this basis. He is not permitted to stay in seclusion with her or look at her in an imperssible manner.

The fact that she is the wife of his brother only makes things worse. ‘Uqbah ibn ‘Aamir narrated that the Prophet said: “'Beware of entering upon women.' A man from the Ansaar said: ‘O Messenger of Allaah, what do you say about an in-law?’ He replied: 'The in-law is death'.” [Al-Bukhari and Muslim]

What it means is that his meeting with her leads to a disaster as him entering upon her is more serious than a stranger entering upon her, and it is easy for sin to occur because people are lenient about a man mixing with the wife of his brother and him being in seclusion with her, so he may enter upon her without anyone expressing disapproval. Therefore, the evil from that is more expected and temptation is more likely. Hence, whoever commits this is obliged to sincerely repent to Allaah. The condition of repentance have been clarified in Fatwa 86527.

Since he committed Zina (fornication or adultery) with the wife of his brother in the past, he needs to be even more careful in dealing with her especially if she is not observing the limits of Allaah, as she may seduce him again. What had happened between them may make her less shameful around him and more daring to commit the same sin again. So, if he visits his brother's house, he should do so in his brother's presence. Any family meetings that do not abide by the Islamic guidelines should be avoided. For more benefit, please refer to Fatwa 92070.

Allaah Knows best.