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.
This man has indeed committed a major great sin in committing incest with his two daughters. It is indeed an abominable act that the natural disposition of a human being rejects. Allaah warned us against committing adultery and/or fornication as He says (interpretation of meaning): {And come not near the unlawful sexual intercourse. Verily it is a fahishah (i.e. anything that transgresses its limits; a great sin) and an evil way (that leads one to Hell) unless Allaah forgives him/her.}[17: 32]. This is in case this abominable act is committed with other women, so it is even more serious and abominable if it is committed with a female relative, that's why Allaah says regarding marrying a female relative (interpretation of meaning): {And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.}[4: 22]. So it is an obligation on whoever is aware that this man has committed this act to remind him about Allaah and the evil fate of this heinous act in this life and the Hereafter, and to urge him to sincerely repent before he dies on this state. He should also be reminded that the Mercy of Allaah is vast and that He accepts the repentance of whomever repents. No matter how great the sin is, Allaah can forgive it. Allaah says (interpretation of meaning): {Say: O! My slaves who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the mercy of Allah; verily, Allaah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful.}[39: 53]. This man has to be careful of Satan making him despaire of the Mercy of Allaah and beautify for him his continuing to sin. All of them (the man and his daughters) and also those who came to know about their sin have to conceal their sin. If it is probable that this man could return to his sin, he should not be left alone with one of his daughters, but rather they should treat him as an alien man (a marriageable man) and should cover themselves in front of him, and he should be threatened to be reported to the authorities who can prevent him from committing this sin.
Finally, we draw your attention to the following two matters:
Firstly, if the daughters get pregnant from this incest, the child will not be named after this man.
Secondly, it is wrong to say that Allaah is not going to forgive this sin. We have previously stated that the Mercy of Allaah is vast, and he even forgives to a non-Muslim if he repents, Allaah says (interpretation of meaning): {Say to those who have disbelieved, if they cease (from disbelief) their past will be forgiven…}[8: 38].
Allaah knows best.