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.
First of all, it should be known that illegal sexual intercourse is among the major great sins and it necessitates repentance. It becomes worse and much abominable if one commits such a sin with an unmarriageable female relative. The punishment of this evil is that the guilty party should be killed according to the most correct opinion of Muslim scholars. For the narration that Al-Baraa Ibn 'Aazib said: "I met my uncle and a banner was with him (heading a troop), I asked: 'Where are you heading to? He replied: 'The Prophet sent me to a person who has married his former stepmother, to kill him and to capture his property." [An-Nasaa'ee]
Muslim scholars have different opinions concerning the matter if a man commits illegal sexual intercourse with his daughter in-law, whether her marriage to his son becomes nullified or not. Some of them are of the opinion that her marriage does not become nullified. This is the opinion of Imaam As-Shaafi'ee. Prior to him it was the opinion of Ibn 'Abbaas Imaam Ibn Al-Qayyim preferred this opinion in his book Zaad-ul M'aad.
The other opinion is that if a father makes illegal intercourse with his son's wife the marriage of the son becomes null, hence the son must separate from his wife. Both opinions are well known in the
However, we are of the opinion that nullifying the marriage of the son to his wife is not compulsory.
Allaah Knows best.