If you've seen Fantastic Beasts: The Crimes of Grindelwald, then you know the ending is filled with about 15 minutes of shocking twists, ending on a reveal so outrageous, the audience stares in stunned silence as the credits roll. If you haven't seen the film, I recommend waiting to read the rest of this article, because I'm about to talk about a reveal which can best be described as having "Snape Kills Dumbledore" level spoilers. Don't say you weren't warned, because this is about to dive into how Dumbledore and Credence are related. Warning: Spoilers for Fantastic Beasts: The Crimes of Grindelwald follow.
Regarding keeping spoilers all the way under wraps, Warner Brothers, and in particular the marketing team behind Fantastic Beasts, should be commended. The film did a remarkable job of misdirection with all the Corvus Lestrange stuff, seeding clues into the trailers and the way cast lists were released. Potterheads went into the film thinking Credence would be revealed as the heir to the Lestrange family fortune, and quite possibly explaining why they were so primed and ready to follow Voldemort a generation on.
Instead, it turned out this was all a bait-and-switch. Corvus Lestrange was dead, the baby died when Leta switched them in a fit of childish behavior on the trip to America just before the boat sank. The baby who she accidentally saved in his place was a secret Dumbledore.
According to Grindelwald, Credence is actually the long-lost younger brother of Albus, and his name is Aurelius Dumbledore.
This is difficult for fans to swallow. After all, Dumbledore is nearly 50 in this film. (He was born in 1881, it's 1927, so he's 46.) Creedence is far younger, a good five or so years younger than Newt Scamander and crew. So how could this be?
It can, thanks to Rowling's timeline being a bit mushy at this point.
The shot of the boat shows Leta has a young girl, but no age is given. She looks to be about 5 or 6. Leta was born in 1897. (She entered Hogwarts in 1908, so this was as early as 1901 but probably not later than 1904.
Percival, Albus father, was sent to Azkaban sometime in the late 1880s after he took revenge on the muggles who attacked Ariana and left her mentally unstable. All Albus says is he died there, but not when.
Kendra died in 1899. There is no fudging on this, the timeline and the Harry Potter wiki are clear on this point. So the chances of this being Kendra's child are nil, but it could be Percival's, via another woman.
If one takes Grindelwald's story as truth, then Aurelius was born to a woman who was either a prisoner along with Percival or someone who visited him while he was imprisoned. Either way, this was a baby with no one to take care of him, and a father in prison for life. It may have been Alberforth knew and decided, considering what happened with Albus, it would be best to get the baby out of the country.
The details won't come out until the next film. Fantastic Beasts 3 arrives in 2020.