All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
What has been done to you by the mentioned person is a big atrocity and a monstrous crime, that is fornication; worse than that, is that he committed this crime with his daughters and his relatives and near ones. Committing fornication with such people is the worst kind of sin and offence. It is greater than committing fornication with other than them.
The Prophet has explained that whoever commits fornication with the wife of his neighbor, that his sin is greater and worse than his committing the same thing with ten women. That is because it is one of the grave major sins.
The Prophet asked his Companions: “What do you say about fornication? They replied: “It is unlawful as Allah and His Prophet have prohibited it and it is unlawful until the Day of Judgement. He said: "It is less serious for a man to fornicate with ten women than for him to fornicate with his neighbor’s wife." [Ahmad]
Besides, ‘Abdullah ibn Mas'ood narrated that a man asked the Prophet saying: “What is the biggest sin in the sight of Allah?” He said, “To set up rivals unto Allah though He Alone has created you.” I said, “In fact, that is a tremendous sin,” and added, “What is next?” He said, “To kill your child being afraid that he may share your food with you.” I further asked, “What is next?” He said, “To commit illegal sexual intercourse with the wife of your neighbor.”
Then the Almighty Allah revealed its confirmation: Allah Says (what means): {And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty.} [Quran 25:68] [Al-Bukhari and Muslim]
That is because it is an obligation on him to protect the dignity of his neighbor as he knows his (i.e. neighbor) entrance and exit. If an evil act is committed by a reliable and relative person, its sin is even greater. How about a person who commits this evil with his own relative? How about a person who commits it with his own daughters? No doubt that his sin is great before Allah and before the people. All that happened to him due to a relapse in his innate character (nature) and crookedness in his behavior, although he behaves with people in a different way. The correctness of this man seems to be hopeless unless Allah wishes it. He is then considered to be abnormal, with a bad and crooked condition.
That is why the text came very clear that whoever happens to commit fornication with his unmarriageable woman, its punishment is: he has to be put to death. The Prophet said: “Whoever happens to fall (in fornication/adultery) with his Mahram (unmarriageable woman), kill him.” [At-Tirmithi]
A case was presented to Al-Hajjaj that a man committed fornication with his own sister forcibly; he said: detain him and ask from amongst the Companions of the Prophet . So they asked ‘Abdullah ibn Mulassif who said: I heard the Prophet saying: “Whosoever transgresses the Muslims, make line on the middle of his body with a sword.” This means divide him into two parts with a sword. This and the people similar to him whose innate character is relapsed, their punishment in Islam is to put them to death for their disgraceful crime with their Mahram women. If Allah does not punish him in this world and he pretends to be pious, he will never be able to escape from Allah's punishment on the Day of Judgement. His honor will be torn apart just as he tore apart the honor of his daughter and his relative, and just as he spread bad rumors about them in order to cover his reality in front of the people. That Muslim sister has to pray to Allah so that He will give her back her right and protect her from evil.
If the Muslims have a chance to prove his guilt in order that he be punished, they should not hesitate to do so, even if they have to prove it before the authority of the country where they settle matters (which might not be a Muslim country) in order to make of him a lesson and a warning for others.
Allah knows best.