[The Ruling on Marrying a Brother’s Daughter Born of Fornication]

Answer—and Allah grants success: If a man fornicates with a woman and that woman bears a daughter from the semen of that very man, it is not permissible for that man—nor for his brother—to marry that daughter. For, in truth and reality, she was created from his semen, even if her lineage is not established in the Sharīʿah. Believers halt at doubtful matters; and in the ḥadīth: “Leave what causes you doubt for what does not cause you doubt.”
And in the ḥadīth from the Prophet (May Allah bless him and his family and grant them peace): “The child belongs to the [lawful] bed, and for the fornicator is the stone.” Its meaning is that the fornicator does not, by fornication, have the lineage of the child established for him, nor does he acquire any right to the child; rather, what he deserves is stoning with the stone.
Nevertheless—though this is so—the legal rulings are built upon outward appearances. If a person is certain that that girl was created from his semen, then it is not permissible for him to marry her, nor for his brother to marry her, nor for anyone to marry her who would be prohibited to him were she his daughter in the legal sense.
What we have stated is what is nearer to the truth, more rightly guided, safer in the religion, and more precautionary for believers
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1