All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

It is established in Islamic Sharia that cheating is forbidden in all aspects of life because Abu Hurairah (RA) narrated:  Allah's Messenger (PBUH) once came upon a heap of grain, and when he put his hand inside it, his fingers felt some dampness. So, he asked: "What is this, O owner of the grain?" He replied: "Rain had fallen on it, O Allah's Messenger." He said, "Why did you not put it (the damp part) on the top of the foodstuff so that people might see it? Whoever cheats has nothing to do with me." [Reported by Muslim].

The general meaning of this Hadith includes cheating in exams as well. As cheating is forbidden, helping in that is forbidden as well because Allah the Almighty Says {What means}: "Help ye one another in righteousness and piety, but help ye not one another in sin and rancour: fear Allah. for Allah is strict in punishment." {Al-Mai`dah/2}. Whoever sees an act of cheating should try to stop it as much as he/she can and it isn't lawful for a Muslim to keep silent about an evil act which he/she can deny. Being silent makes him/her a partner in this sin (Cheating).

In conclusion, your reporting that person sitting for the exam instead of his brother was fulfillment of a moral, professional and legal duty. The idea behind an exam is distinguishing the hardworking student from the lazy. This is in addition to the fact that many rights are based on the point scored in it. Therefore, if the invigilator did nothing about the above violation then he is considered a partner in sin and betrayal of trust as he shouldn`t be lenient or turn a blind eye on acts of cheating that take place in the exam room. This is in order to prevent cheating from spreading in society, then, God forbid, the Muslim Ummah (Nation). And Allah the Almighty Knows Best.