His wife and son have regular incestuous relations together
28-10-2013 | IslamWeb
Question:
Respected Sheikh, I am a man in my late sixties and recently discovered that my 33 year old wife has regular incestuous relations with my 17 years old son. When I confronted them they denied it, but one day I came home unexpectedly and found them in the act. They repented and promised not to do it again. I am devastated and don’t know what to do. I beg you to tell me in the light of the Quran and hadith what course of actions I shall take.
Answer:
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.
If what you mentioned in the question about your wife and your son is true, then they have committed an abominable and obnoxious sin, as Zina (fornication or adultery) is a grave major sin. This sin is even graver when it happens between the Mahaarim (unmarriageable relatives) as your wife is eternally forbidden to your son (for marriage). Allaah says (what means): {And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allaah] and was evil as a way.}[Quran 4:22] However, if they had both truly repented, then they did well.
Now, if your wife becomes righteous and shows good conduct, then you should keep her and have good marital relations with her and do not hold her accountable for what she had done, but you are obligated not to have sexual relations with her until you make sure that she is not pregnant from Zina by the passing of one menstrual period.
Also, you should be keen on disciplining both of your wife and son and teaching them the matters of their religion as what they did is mostly due to ignorance and negligence. Allaah says (what means): {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allaah in what He commands them but do what they are commanded.}[Quran 66:6] Ibn ‘Abbaas 
said (while interpreting the above verse): “That is to say to discipline them and teach them.”
On the other hand, if you are suspicious about your son in the future, then you should take the matter firmly in hand and prevent him from being in seclusion with your wife and prevent her from appearing in his presence without Hijab. Likewise, if you are suspicious about your wife, then there is no good in keeping her as your wife; rather you should divorce her so that she would not contaminate your bed and give birth to children who are not from you.
Finally, it should be noted that committing Zina is generally due to being lenient in matters that lead to it, like a woman appearing in front of men (other than her husband) in an inappropriate manner, like wearing revealing clothes and so forth. So, one should be careful about this.
Allaah Knows best.