Is it permissible for me to repel an assault on my brother or sister, or must the one being assaulted be the one to repel the assault?

The one who has the right to retribution (Qisas) or forgiveness is the victim alone, because he is the owner of the right, and it is not permissible for anyone else to claim it. Retribution (Qisas) can only be carried out by empowering the competent authorities, such as Sharia (definition) courts and judges. It is not permissible for an individual to carry it out himself, as there is no guarantee against injustice and aggression in such a case. Some scholars have gone so far as to prevent the ruler from enabling the victim to carry out retribution himself in cases less than homicide. This is the view of the Malikis and Shafi’is, and a statement among the Hanbalis, for fear of an offense that cannot be remedied. As for insults and curses, the wronged person may defend himself by responding to evil with its like, though forgiveness is better, for Allah Almighty says: {The recompense of an evil is an evil like thereof. But whoever pardons and makes reconciliation, his reward is due from Allah. Indeed, He does not like wrongdoers}.