She did something haraam with her husband’s brother
Inna Lillaahi wa inna ilayhi raaji’oon (‘Truly, to Allaah we belong and truly, to Him we shall return’).
Your husband has fallen into that which the Prophet (peace and blessings of Allaah 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 Allaah 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. Allaah 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 Allaah 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-Allaah ibn ‘Umar (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Avoid these filthy things (sins) that Allaah has forbidden, and whoever does them let him cover himself with the cover of Allaah and repent to Allaah, for whoever tells us about his sin, we will carry out (the punishment ordained by) the Book of Allaah 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 Allaah (peace and blessings of Allaah be upon him) when he was in the mosque, and called him, saying, “O Messenger of Allaah, I have committed adultery.” The Messenger of Allaah (peace and blessings of Allaah 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 Allaah, I have committed adultery.” The Prophet (peace and blessings of Allaah 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 Allaah, I have committed adultery.” When he had testified against himself four times, the Prophet (peace and blessings of Allaah be upon him) called him and said, “Are you crazy?” He said, “No, O Messenger of Allaah.” He said, “Are you married?” He said, “Yes, O Messenger of Allaah.” 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 Allaah and cover yourself with the cover of Allaah.” 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 Allaah 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 Allaah (peace and blessings of Allaah 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 Allaah 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 Allaah 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 Allaah has covered her.
And Allaah knows best.