Praise be to Allah.
Undoubtedly this is a revolting and abhorrent action that is unacceptable to sound human nature and decent souls, let alone the soul of the people of Islam and faith who regard zina with a woman who is not a relative as abhorrent, so how about if it is with one who is a mahram? How about with the man’s daughter who came from his loins?
By Allah, the calamity that has befallen his sons is a great calamity and we ask Allah to help them and to grant the Muslims well-being.
Despite what this father has fallen into of degenerate behaviour, he still has a father’s right to kind treatment and upholding ties of kinship, whilst also fulfilling the duty to Allah, may He be exalted, of offering advice, denouncing the evil and preventing that from happening again in the future.
If the sons advise him and he shows signs of repentance and regret, or his being thrown out of the house has the effect of causing him to repent and regret what he did, then the sons have to honour him and pay attention to his rights, whilst taking precautions with regard to the daughter’s (or daughters’) interactions with him. They should not be careless about uncovering in front of him, and he should not be allowed to be alone with one of them. The sons should also bear in mind that sin is always possible for the son of Adam, but no matter how great the sin, if the person repents from it and mends his ways, Allah will accept his repentance.
But if he gives no sign of having repented and regretted it, and there is the fear that if he stays in the house he will continue to be tempted, then the sons have to give him accommodation in a separate house, and they should treat him with kindness as is required of them, or the daughter should go and live with one of her trustworthy mahrams, whilst the father stays in his house. That is so as to ward off his evil, whilst still giving him his rights.
See also the answer to question no. 46886
We ask Allah to set the affairs of the Muslims straight.
And Allah knows best.