What is the lawful course of action for me towards a man who is flirting with my sister? Is it permissible for me to discipline him by striking him? And what is the ruling if the matter escalates to killing? What should I do with the rest of the Muslim women?

Jealousy over one's honor and Allah's sacred injunctions is commendable, and the absence thereof is blameworthy. Ill-mannered speech is objectionable and should first be denounced verbally and courteously, then by hand if possible and if it does not lead to a greater evil. If he attempts to assault you, he is an aggressor and should be repelled with the easiest means available, then progressively with more forceful means. It is not permissible to strike him merely for a dispute, nor to kill him merely for attempting to kill you.