What is a god but man wielding the force of Chaos? To him nothing is true; everything is permitted. There is no purpose in his existence; he is free to choose his own. He has bound himself to earth forever and reincarnates at will.
Your story is a perfect example of exactly why gaslighting is abuse. If this boy did exist in your family, and I think it's obvious at this point that he did, your family have all made you doubt not just your memories of him but also your ability to perceive and trust reality. This is deeply psychologically harmful. To be lied to for whatever reason that caused you to lose confidence and security in your own perception of the world. No child should ever have those kinds of fears.