Is it permissible for the son of a fornicator to marry the daughter of his father from fornication, knowing that the semen of fornication is not respected by the Shafi'is?

Scholars have differed regarding the ruling on marrying relatives born from adultery. The majority are of the opinion that it is not permissible, while others, such as Malik and Shafi'i, in the well-known position of their madhhabs, permitted it. Shaykh al-Islam [Ibn Taymiyyah] broadly stated the difference of opinion concerning marriage to a woman with whom the father committed adultery after the father's death. Abu Hanifa, Ahmad, and one of Malik's two opinions held it to be forbidden, while Shafi'i and the other opinion of Malik permitted it.

Prohibition in marriage does not imply a mahram relationship that permits seclusion (khalwa) and looking (nazar). Adultery does not establish a mahram relationship nor does it permit looking.