Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- You must hasten to put an end to this relationship, and keep yourself busy with useful and beneficial things; it may be that because of this relationship, you have been deprived of the joy of doing righteous deeds, or some of them.
2- You have to pray, and find joy in conversing with Allah, remembering Him and reciting His Book, and you have to offer a lot of supplication, asking Him to accept your repentance and forgive you.
Answering your question, the Fatwa Center at Islam Q and A, states:
We are very happy to see that you want to put a final stop to this relationship, and we ask Allah, may He be exalted, to enable you to do that.
No one would hesitate to regard such relationships as haram. It is sufficient to note that the one who does that feels that he or she is doing something wrong and hides it from people, and is not able to show it openly in front of people. This is sufficient to indicate that such actions are haram.
The Prophet (peace and blessings be upon him) said: “…sin is that which wavers in your heart and you do not want the people to find out about it.” (Muslim)
These haram relationships lead to a number of negative outcomes, such as the following:
1- Simply doing something that is haram. This is a grave evil that has an impact on one’s spiritual wellbeing, as do all sins.
2- If such sinful relationships become known, that leads to the girl having a bad reputation which will overshadow the good qualities that she has, because people will only remember these misdeeds on her part, and you know that if a girl becomes well-known for such things, how people will treat her and how grave her loss will be.
This may lead to her falling into that which is even more serious, and she may regret it at the time when regret will be of no benefit.
It is strange that every woman who falls prey to such things says, “I trust myself and I trust the man I am talking to; we are not like other young people…”
There are many, endless stories, but those who learn from them are few.
You should understand that we see you as hurtling towards death without realizing; others may be moving more rapidly than you towards doom, but we fear for you as we would fear for our own daughters and sisters.
There is no need for a gradual approach in order to put an end to this haram relationship; that idea may come from the deceit of Satan. Rather the believer, man or woman, as soon as he or she finds out that this action is haram, has no other choice but to give it up. Allah (may He be exalted) says:
{It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allah and His Messenger has certainly strayed into clear error.} (Al-Ahzaab 33:36)
{The only statement of the [true] believers when they are called to Allah and His Messenger to judge between them is that they say, “We hear and we obey.” And those are the successful. And whoever obeys Allah and His Messenger and fears Allah and is conscious of Him – it is those who are the attainers.} (An-Nur 24:51-52)
We ask Allah to enable you to repent and to accept it from you.
You said that this man is married, which means that you are wronging your sister (his wife) by means of this relationship, because undoubtedly he is giving you a share of his time, kind words and affection, when his wife and children are more deserving of that than you, so you have usurped their rights.
Moreover, you may be a cause of the spoiling of the relationship between him and his wife and children.
Furthermore, think about yourself; if this man were your husband, would you like him to have such a relationship with another woman?
If you would not like that, then why do you like that for your sister?
You must hasten to put an end to this relationship, and keep yourself busy with useful and beneficial things; it may be that because of this relationship, you have been deprived of the joy of doing righteous deeds, or some of them.
You have to pray, and find joy in conversing with Allah, remembering Him and reciting His Book, and you have to offer a lot of supplication, asking Him to accept your repentance and forgive you.
Almighty Allah knows best.
Source: www.islamqa.info