What is the punishment of a brother who has sex with his sister? And how can he be forgiven?

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.

Indeed, it is a painful disaster to have a calamity from the one whose side should be safe and secure. What a misfortune for one who is responsible for closing the door of evil to open it himself. In this case the guard who is supposed to be an honest person turns to be a criminal. The reason for all these problems is being far from applying the rulings of the Sharee’ah. The Prophet ordered the beds of children when they reach the age of ten years to be separated, since this age is the beginning point of inclination to the opposite sex.

The Prophet said: “Order your children to perform the prayer when they are seven years of age and beat them if they neglect to do so when they are ten years of age, and separate between them in beds (when they reach ten).” [Ahmad and Abu Daawood]

Allah Says (what means):{O you who have believed, let those whom your right hands possess and those who have not [yet] reached puberty among you ask permission of you [before entering] at three times: before the dawn prayer and when you put aside your clothing [for rest] at noon and after the night prayer. [These are] three times of privacy for you.} [Quran 24: 58]. In this verse Allaah has ordered the children to ask permission lest they could see something which could arouse their sexual desire.

The end result is that a guardian is responsible to teach the children Islamic ethics and take care of them. He should not bring bad and erotic objects at home, as this badly affects the children's morals. We advise our brother to do the following:

1. Exhort them both: the brother and the sister. Explain them that they have committed a grave sin. The punishment of this crime in Islam is to execute the one who commits it. The Prophet said: “Whoever has sex with a Mahram (an unmarriageable woman) you should kill him”.

2. You should not give any chance for them to be alone in any place.

3. Each one of them should be married as early as possible.

4. Urge them to fast because fasting diminishes one's sexual power and helps to lower the gaze. Keep them away from all erotic materials such as pictures, magazines etc. The Prophet said: “O young people! Whoever can marry should marry, as it helps to lower one’s gaze, and protects one’s private parts, and whoever is not able to marry should fast, as this helps him to protect his private parts (from committing illegal sexual intercourse).”  [Al-Bukhari and Muslim]

Allah knows best.