What is the ruling on someone who saw his sister kissing a foreign man in an indecent manner for five seconds, and she was unaware that he saw her? Should he conceal her, based on the Prophet's (peace and blessings be upon him) saying: "Whoever conceals a Muslim, Allah will conceal him in this world and the Hereafter"?

Concealing the sins of disobedient people who do not openly proclaim them is a virtue, especially when it combines the right of Islam with the right of kinship. Scholars have unanimously agreed on the desirability of concealing the flaws or sins of a Muslim whose faults one comes to know, provided that individual is not notorious for corruption. This is based on prophetic hadiths that encourage covering a Muslim's faults, such as his (peace be upon him) saying: "Whoever conceals a Muslim, Allah will conceal him on the Day of Resurrection." This is also because revealing such faults can lead to forbidden backbiting and the spread of indecency. Along with concealment, advice should be offered, and the disobedient person should be admonished and warned about Allah Almighty.