He does not pray and they are afraid that he is working as a pimp
Question

                I found out that my sister’s husband earns his money through haram activities. He brings women from their country where to the gulf countries to work as prostitutes.  He also commits adultery with them, he confessed to my other sister’s husband. I told my father but he did not believe and said that he is a good man and said also that my other sister’s husband is jealous of him and this is why he is lying about him. But I believe my other brother in law, because my first brother in law is always out, especially at night. My sister told me something; she doubts that he is betraying her. He lies to her and she believes him. She is pregnant now, and has a daughter. I did not tell my sister what I heard about her husband, I want to know my duty towards my sister. For two years since she got marries to him, she suffers from psychological diseases. He does not pray, nor does he fear Allah, and Allah knows best.

Please tell me what should I do in this situation? Should I disclose his matter and tell my sister or shall I keep it a secret? I always ask Allah to guide him.

What worries me the most is my fear. I fear he gets a dangerous disease and passes it unto my sister. I fear her children will be negatively affected.
Praise be to Allah.
    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.