She did something haraam with her husband’s brother

Inna Lillaahi wa inna ilayhi raaji’oon (‘Truly, to Allah we belong and truly, to Him we shall return’). 

Your husband has fallen into that which the Prophet (peace and blessings of Allah be upon him) warned us against when he warned men against entering upon (non-mahram) women. It was said to him, “What about the in-law?” He said, “The in-law is death.” The word hamw (in-law) refers to the husband’s brothers and other relatives such as cousins (sons of paternal uncle). What is meant by the Prophet (peace and blessings of Allah be upon him) saying “The in-law is death” is that there is more to be feared from him than from anyone else because he is able to reach the woman and be alone with her without anyone denouncing him for that, because no one will find it strange that he enters the house. How often we hear of regrettable incidents that occurred because of the husband’s brothers entering upon their brother’s wife, even cases of adultery and the wife becoming pregnant by the husband’s brother. Allah is the One Whose help we seek. 

It is not permissible for your husband to seek out the past and try to unearth bad things. Rather he should cover up whatever Allah has concealed, especially after repentance from such things, because his heart will never be clear again after that, and he will think of everything you do after that as being of that nature. 

It was narrated from ‘Abd-Allah ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Avoid these filthy things (sins) that Allah has forbidden, and whoever does them let him cover himself with the cover of Allah and repent to Allah, for whoever tells us about his sin, we will carry out (the punishment ordained by) the Book of Allah on him.” 

Narrated by al-Haakim in al-Mustadrak ‘ala al-Saheehayn, 4/425; al-Bayhaqi, 8/330. Classed as saheeh by al-Albaani in Saheeh al-Jaami’, 149. 

It was narrated that Abu Hurayrah said: A man came to the Messenger of Allah (peace and blessings of Allah be upon him) when he was in the mosque, and called him, saying, “O Messenger of Allah, I have committed adultery.” The Messenger of Allah (peace and blessings of Allah be upon him) turned away from him. The man came to the side of his face that he had turned away from him and said, “O Messenger of Allah, I have committed adultery.” The Prophet (peace and blessings of Allah be upon him) turned away from him again, and the man again came to the side of the Prophet’s face that he had turned away from him and said, “O Messenger of Allah, I have committed adultery.” When he had testified against himself four times, the Prophet (peace and blessings of Allah be upon him) called him and said, “Are you crazy?” He said, “No, O Messenger of Allah.” He said, “Are you married?” He said, “Yes, O Messenger of Allah.” He said, “Take him and stone him to death.” 

Narrated by al-Bukhaari, 6430; Muslim, 1691. 

In some reports it says that a man who had become Muslim came to Abu Bakr and told him that he had committed adultery. He said, “Repent to Allah and cover yourself with the cover of Allah.” Then he came to ‘Umar likewise. See Fath al-Baari, 12/125.

 Al-Haafiz ibn Hajar said: 

From this case we learn that it is mustahabb for anyone who finds himself in a similar situation to repent to Allah and to conceal his action and not mention that to anyone, as Abu Bakr and ‘Umar indicated to Maa’iz. 

Whoever finds out about any such thing should conceal it because of the things that we have mentioned and should not expose it or refer it to the ruler, as the Messenger of Allah (peace and blessings of Allah be upon him) said in this story, “If you had concealed him with your garment [i.e., told him to repent and keep quiet], that would have been far better for you.” Hence al-Shaafa’i confirmed this idea and said: If a person commits a sin and Allah conceals it, I prefer for him to conceal himself and repent, and I quote the story of Maa’iz with Abu Bakr and ‘Umar as evidence. 

This story shows that it is mustahabb for the one who commits a sin and then regrets it to hasten to repent from it, and not to tell anyone else about it. He should cover himself as Allah has covered him. If it so happens that he tells someone, it is mustahabb for that person to tell him to repent and conceal that from the people, as happened with Maa’iz and Abu Bakr then ‘Umar. 

Fath al-Baari, 12/124, 125 

Based on this: 

The man has no right to seek out information on the past from which his wife has repented, for the reasons outlined above. And the women should not tell her husband of what happened in the past that she has repented from; she should cover herself as Allah has covered her. 

And Allah knows best.