What is the ruling of the Sharia concerning a man who looked at his sister with lust while she was unclothed, and what is the expiation for this sin, and how can he redress the injustice to his sister, and will his repentance be accepted?

It is absolutely impermissible to look at the 'awrah (private parts) of anyone other than one's wife, based on the saying of the Prophet (peace be upon him): "A man should not look at the 'awrah of another man, nor should a woman look at the 'awrah of another woman." The prohibition and sin become more severe if the person being looked at is a mahram (unmarriageable kin), because this contradicts natural disposition (fitra) and ethics.

Moreover, merely looking with lust at anyone other than one's wife is forbidden, even if it is at something less than the 'awrah. Whoever has done this must repent to Allah and frequently seek forgiveness and supplicate for his sister. It is not necessary to inform her of what occurred, in order to avoid a greater harm.

Indeed, Allah is All-Generous, All-Merciful; He accepts repentance from His servants and forgives evil deeds. Therefore, one should not despair of His mercy, for despair is another sin. Nothing else is entailed by what happened other than sincere repentance.