What we advise you to do with regard to this problem is three things: make sure of what your sister’s husband is actually doing, advise him if it is proven that what he is doing is evil deeds, and strive to have the marriage annulled if he persists in doing it, whether it is committing adultery or not praying. 

With regard to the first issue: 

It is known that the basic principle concerning a Muslim is that he is innocent, and it is not permissible to accuse him of something that is not true, otherwise the one who is making the accusation is guilty of sin. 

Allaah says (interpretation of the meaning):

“And those who annoy believing men and women undeservedly, they bear (on themselves) the crime of slander and plain sin”

[al-Ahzaab 33:58]. 

It is essential to establish the truth of anything you hear before basing any decision or ruling on it. Allaah says (interpretation of the meaning):

“O you who believe! If a Faasiq (liar — evil person) comes to you with any news, verify it, lest you should harm people in ignorance, and afterwards you become regretful for what you have done”

[al-Hujuraat 49:6]. 

Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allaah have mercy on him) said:  

This is another etiquette which should be adopted and followed by those who have understanding, which is that if an evil person tells them something, they should verify his report and not just accept it willy-nilly, because there is grave danger in that and the possibility of falling into sin. If his report is treated in the same manner as the report of an honest person of good character and is acted upon, then it results in unlawful loss of life or wealth because of that report, it will be a cause of regret. Rather if news comes from an evil person, it must be checked and verified. If evidence and circumstances indicate that it is true, then it may be acted upon and believed, but if it indicates that it is false, then it should be disbelieved and not acted upon. This indicates that the news of an honest person is acceptable and the news of a liar is to be rejected, and the news of an evildoer is to be examined further, as we have said. 

Tafseer al-Sa’di (p. 799). 

The basic principle is not to accuse people, and that people are to be presumed innocent. The lie may come from one who claims that the person admitted it to him, and this is not far fetched. If it is proven that the one who brought the news is telling the truth, and that this is what is really happening, then we move on to: 

The second issue: 

Which is advising and exhorting. 

It was narrated from Tameem al-Daari that the Prophet (peace and blessings of Allaah be upon him) said: “Religion is sincerity.” We said: To whom? He said: “To Allaah, to His Book, to His Messenger, and to the leaders of the Muslims and their common folk.” Narrated by Muslim (55). 

It was narrated that Jareer ibn ‘Abd-Allaah said: I gave my oath of allegiance to the Messenger of Allaah (peace and blessings of Allaah be upon him), pledging to establish regular prayer, pay zakaah and be sincere towards every Muslim.” Narrated by al-Bukhaari (501) and Muslim (56). 

That is based on the condition that the advice be given in the best manner to correct faults.  

Allaah says (interpretation of the meaning):

“Invite (mankind, O Muhammad صلىالله عليه وسلم) to the way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Qur’aan) and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His path, and He is the Best Aware of those who are guided”

[al-Nahl 16:125]. 

The best thing you can advise him to do is to pray. It is essential for him to understand that not praying is kufr that puts one beyond the pale of Islam, and that if he dies when he is not praying then he will have died a death of Jaahiliyyah, and will have died as an apostate. Then after that he should be advised to give up the haraam actions such as immoral and evil deeds with those female servants, and pimping them to others. Bringing in female servants in the first place, even if the aim is for work only, involves many reprehensible matters. We have discussed previously the issue of female servants and the ruling on bringing them in from their countries and the haraam things into which people fall when female servants work in their homes. This may be read in the answer to question no. 26282. 

If what has been said about him is proven to be true, and he does not respond to advice and he persists in not praying and in doing evil deeds, then what you must do is: 

The third issue: 

Which is: separating him and his wife by annulling the marriage contract, because the one who does not pray is an apostate, and his marriage contract with a Muslim woman is to be annulled. Moreover it is not permissible for a chaste woman to remain married to an immoral adulterer. His not praying dictates that the marriage contract should be annulled; as for his committing immoral actions, it does not dictate that the marriage contract be annulled, but her acceptance of his actions makes her a partner with him in them. Such a person cannot be trusted with a daughter or a wife, and there is no guarantee that he will not transmit deadly diseases to her. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said: 

Not praying is kufr which puts one beyond the pale of Islam. If he has a wife, then his marriage to her is annulled, meat slaughtered by him is not permissible, his fasting and charity will not be accepted, and it is not permissible for him to go to Makkah and enter the Haram; if he dies it is not permissible for him to be washed or shrouded, or the funeral prayer to be offered for him, or for him to be buried with the Muslims. Rather he should be taken out into the desert, and a ditch dug for him, and he should be buried into it. If a person’s relative dies and he knows that he did not pray, it is not permissible for him to deceive the people by bringing him to them for them to offer the funeral prayer for him, because offering the funeral prayer for a kaafir is haraam, as Allaah says (interpretation of the meaning): 

“And never (O Muhammad صلى الله عليه وسلم) pray (funeral prayer) for any of them (hypocrites) who dies, nor stand at his grave. Certainly they disbelieved in Allaah and His Messenger, and died while they were Faasiqoon (rebellious, — disobedient to Allaah and His Messenger صلى الله عليه وسلم)”

[al-Tawbah 9:84]  

“It is not (proper) for the Prophet and those who believe to ask Allaah’s forgiveness for the Mushrikoon, even though they be of kin, after it has become clear to them that they are the dwellers of the Fire (because they died in a state of disbelief)”

[al-Tawbah 9:113]

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (12/question no. 26) 

See also the answers to questions no. 10094, 2182 and 5208 

But we reiterate and remind you not to rush to say such things about him or to tell your sister and ruin her life and break up her family without legitimate proof. Our concern about the sacred limits if Allaah being transgressed should be greater than our fear of disease being transmitted or our being offended by marital betrayal. She should strive to make du’aa’ for him and ask that he be guided, and that your sister set her affairs straight, and ask Allaah to protect her and her off spring from his evil and from the evil of all evildoers. 

And Allaah knows best.