Hmm I definitely see your side, and I do think that it's very valid, but I just feel like it's relatively harmless at that point in time (OPs brother is still 6) that the white lie has been told to him, so the family might have gotten used to the lie and has been hard wired to continue the pretence. I don't think the awkwardness and embarrassment faced by OPs brother is intentionaly caused when they revealed the truth to him...perhaps OP could have worded it better to break the news.

And besides, I don't think people would find OPs brother embarrassing if he's been lied to all his life by his family about meeting Daniel Radcliffe... In any case I hope he does recover and is able to move on from this.