I am functioning mostly as a Khateeb in a small town in the United States. The Imaam had to travel overseas and so I have been put in charge by the desire of the local Shura, I have very limited knowledge but nonetheless I base everything on the Quran and authentic Sunnah and the understanding of those scholars who have not fallen victim to Bid'a. I have been asked a question by one of the local people here.
A woman, and her two brothers, have discovered that their brother-in-law, has committed adultery against their sister. The sister does not know and they haven't told her. I told her that they did good by covering the faults of a Muslim as the Hadeeth says. This brother-in-law also is taking money from the business they all share in to pay money to the woman he had been committing adultery with. Their question to me concerned confronting the brother-in-law with the issues of adultery and stealing. I told them to pray Istikhaara and I would pray it as well. Then I realized that the brother-in-law could give a disease to his wife. So I told them that they should confront him soon and if he does not change then they have no choice but to tell the wife. The sister told me that they know he has stopped committing adultery but he is still paying the other woman money, this is the money he is taking from their business.
They then asked me if they confront the husband, and he stops paying the other woman, that the other woman may tell his wife (who doesn't know) about the adultery as a way of causing trouble. So their next question to me was "can they lie to the sister to protect the reputation of the husband?" This was so that their sister never knows what her husband did.
I told her that I did not have the knowledge to answer such questions but I know there are good scholars who can Inshaa' Allaah.
So can you please give some advice that I can give to the sister and her brothers. Just to let you know, this family owns stores where people can buy alcohol, pork, gambling and pornography. I told them they need to get rid of these stores because people who deal in such things, this is how Allaah brings misery to their lives.
Whatever it said that is good is from Allaah and the mistakes are from me and I seek refuge in Allaah.

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We highly commend your keenness on helping other Muslims know about their Religion, and pray to Almighty Allaah that He reward you. Yet, we would like to warn you not to say anything concerning the Religion of Allaah without adequate knowledge.  You should call people only to the things of which you are fully aware. If someone asks you about something beyond your knowledge and you cannot ask knowledgeable people (scholars) about it, then you have to refrain from giving any answer. Performing Istikhaarah prayer (prayer for seeking guidance) is not enough for you to issue a Fatwa permitting or prohibiting something.

As a principle, a Muslim is innocent until proven otherwise. Therefore, if it is completely confirmed that this man commits adultery and theft, his sins have to be concealed and he has to be kindly and wisely admonished. If he repents, then thanks to Allaah, but in case he insists and does not give up such sins, and there is no hope of change, then his wife has to be informed and advised to seek divorce, since there is no good in living with a thief and adulterer.

This kind of marital life could have dangerous effects on her and her children, since the father could be a reason for their delinquency.

The woman with whom he practiced adultery should also be admonished and urged to repent. The family also should be advised to give up trading in prohibited things, since such sinful deeds can be the actual reason behind all the calamites that befall them. Allaah Almighty says (what means): {And whatever strikes you of disaster — it is for what your hands have earned; but He pardons much.}[Quran 42:30].

Allaah Knows best.