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.