What is the ruling of Islamic law concerning dealing with a 24-year-old sister who repeatedly deviates and fails to commit, and threatens to run away if she is not returned to the city where she deviated, causing severe distress to her family and fear for her mother who has threatened suicide? And is it permissible to kill her if she is a "devil from mankind," by analogy with the action of Al-Khidr?

It is impermissible to kill one's sister, regardless of her actions, because an evil cannot be remedied by a greater evil. Killing is a grave sin, and its perpetrator is threatened with Hell, the wrath of Allah, His curse, and a tremendous punishment. It is the first matter to be judged among people on the Day of Resurrection. A believer remains righteous as long as they do not shed unlawful blood. The perishing of the world is less significant to Allah than the unjust killing of a believer.

Instead, one should be gentle with her, advise her kindly, treat her with benevolence, pursue effective methods of دعوت (invitation to Islam), provide Islamic tapes, books, and pamphlets, pray for her guidance, connect her with righteous company, and help her marry and form a stable family.

As for the action of Al-Khidr (peace be upon him) in killing the boy, it was by revelation from Allah. It is not permissible to use this as an argument for killing someone who commits an act that incurs a legal punishment (حد). The establishment of legal punishments (إقامة الحدود) is the prerogative of the ruler (ولي الأمر), not individuals.