For better or worse, Fantastic Beasts: The Crimes of Grindelwald vastly complicates the universe that Harry Potter fans know and love. Albus Dumbledore, a big part of the story that develops through Fantastic Beasts, gets a thornier story arc as the prequel series explores and expands on his history with Grindelwald and his potentially secret brother.
The biggest reveal about the future Hogwarts headmaster comes when it is disclosed that the two wizards and former friends have a tough-to-break blood pact in which they both swear to never fight each other. If we consider this in the context of the Harry Potter series, the narrative around Dumbledore's family gets a little dicey and confusing. After all, shouldn't the pact have prevented the duel that tragically kills Dumbledore's sister Ariana? Well, possibly. But it could've been the reason she died as well.
Let's rewind to Harry Potter and the Deathly Hallows. We know from the book and the movies that Grindelwald and Dumbledore have a very close relationship in the Summer of 1899. They obsess over the elder wand, the most powerful wand known to wizards, and even plan for a new world order between Muggles and wizards. But their relationship ends, presumably as Grindelwald's plans get more extremist. The now-dark wizard eventually casts the Cruciatus curse on Albus's brother Aberforth, and when Albus tries to protect his brother, a three-way duel ensues. Albus's troubled sister Ariana intervenes and dies in the crossfire.
Here's where things get complicated. After The Crimes of Grindelwald premiered, there have been many theories surrounding Ariana's tragic destiny. If Dumbledore made the pact with Grindelwald before the duel, technically the two would have been prevented from fighting, and that in turn might have been the cause of Ariana's death. A theory on Nerdist speculates that breaking a blood pact by fighting could have had grave consequences. As Dumbledore and Grindelwald tried to fight, one of their spells could've bounced off and fatally hit Ariana.
On the other hand, Grindelwald, bounded by the blood pact, could've chosen to go after Dumbledore's siblings out of spite. The dark wizard may have been intimidated by Ariana, who was likely a powerful Obscurial. It's curious to note that when Dumbledore drinks Voldemort's potion in Harry Potter and the Half-Blood Prince, he begs someone in his vision to hurt him instead of "them." The theory on Nerdist contends that the puzzle pieces fit together if the Hogwarts headmaster knew he couldn't break his blood pact on the night his sister died.
Then, there's the chance that Dumbledore entered the pact after his sister died. There's a chance that after Ariana's death, Dumbledore and his old friend regret the tragic situation and make a truce to never fight again, but the theory seems less likely.
There's no doubt that a future Fantastic Beasts movie will break down everything that happened on that ill-fated night to the bare bones — pun intended, since Credence Barebone, or Aurelius Dumbledore, will most likely be relevant to untangling the Dumbledore family tragedy.