If some one had physical relationship with both mother and daughter at same time but they are not aware about it. But he has stopped making physical relation with mother any more. Now the question is that can he do nikah with daughter legally? He has told to the daughter he had relation with her mother in past but still the daughter wants to marry with him, and she says if you ask Allah the Almighty for forgiveness He will forgive you, if you do not repeat your sin again. Is that possible as per the Shariah to have nikah with the daughter?
One is not lawful to do nikah with the daughter of such a woman with whom one has had physical contact. A daughter becomes haram forever by having physical contact with her mother.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband