Fantastic Beasts: The Crimes of Grindelwald has confused the heck out of the Potter fandom. It's not just the idea of a wholesome witch like Queenie siding with Grindelwald, the moral equivalent of Wizarding World dictator. It's not just the discovery Nagini, who would go on to be Voldemort's right-hand snake, is vehemently against Grindelwald's policies. It's the extra Dumbledore J.K. Rowling threw into the mix. The latest question shows just how bewildered fans are, prompting everyone to ask: Is Aurelius in the Harry Potter books?
Listen, if Dumbledore had even had the barest hint of a secret sibling, don't you think the legions of fan-fiction writing Potterheads would have caught this by now? If there had been even the slightest bit of canon evidence in the novels or the movies he existed, they would have written about 600,000 LiveJournal entries about him.
The fact that no Potterhead ever even considered there might be an extra Dumbledore sibling to use as a character in fan fiction says very clearly there was nothing in the books to suggest this... until now. It's part of why the introduction of Credence Barebone as some long-lost hidden Dumbledore brother was such a complete shock to the system.
To be clear, even the whole piece of "conclusive evidence" of the phoenix coming to Aurelius in his time of need never once came up in the Harry Potter books either. Yes, Dumbledore has Fawkes. (And considering the way everything else is going in these films, one can assume the phoenix with Aurelius now is actually going to be christened Fawkes and go home with Dumbledore at the end of the fifth movie.)
But Aberforth has no pet phoenix. And what about Ariana? That girl was *in need* after she was attacked by Muggles and her magic was damaged, and no birds came to her aid. This piece of evidence is literally introduced by Dumbledore for the first time in The Crimes of Grindelwald, and it's obvious Rowling threw it in there to foreshadow the phoenix coming to Credence. This way there is secondary evidence to confirm Grindelwald's statement Credence is a Dumbledore. Otherwise, every last Potterhead would have snorted Grindelwald was lying and consider the matter closed.
But Rowling put a clever explanation into her story for why Aurelius does not exist in the Potter books at all. It's in his name. Marcus Aurelius was the last of Rome's "Five Good Emperors" and is overlooked a lot in the history books in favor of those who came before and after. Ambrosius Aurelianus was a British King who ruled during the time of Merlin and Arthur and mostly gets left out of the histories.
By naming her new Dumbledore after men who get written out of history, Rowling is making an excuse for why no one ever had a hint of Aurelius Dumbledore before. Now it's just a question of if she can convince fans to accept him in Fantastic Beasts 3, which I am tentatively subtitling Hey Look, More Dumbledores.