About him being Aberforth's son. I don't know, that felt kind of anti-climactic and strange to me. It also really made no sense to me why Grindelwald would lie about Credence being Dumbledore's brother instead of just telling the truth? What difference does it make whether he is his brother or nephew?

I also feel like it was way too easy for Albus to reverse years of manipulation by Grindelwald. It was all resolved too easily, too quickly...

https://www.youtube.com/watch?v=Tg9BT3gJ7eU

No spell can truly bring back the dead...

---

Or if you would rather read:

Crimes of Grindelwald seemingly editted (if not just retconned) a bit of Harry Potter canon, but the biggest change to the history it tries to push is that Albus has a second younger brother: Aurelius Dumbledore.

Which makes absolutely no sense.

For one, Dumbledore's mother died in 1898, and Credence wasn't born till 1904. For another, his father was sent to Azkaban around 1892, and died before getting out.

Not to mention the end of the movie implies him being a Dumbledore makes him extremely powerful, but, if we've learnt anything from the HP series, magical prowess is not hereditary. The Gaunts weren't particularly powerful, most pure-bloods are pretty average, and Muggleborns like Hermione and Lily can be extremely powerful.

So short of a another retcon, there's not really a traditional way Credence could be Dumbledore's younger brother.

( I debunk other theories in the video. )

So with biology and math failing us, is there another way for Credence to be Dumby's younger bro?

While Secrets of Dumbledore may just retcon Aurelius into the story, we can use what we know about magic to put together a possible recipe for building a brother...

Firstly, Albus had extensive experience in the field of Alchemy, working with Nicholas Flamel. This means that he both had access to the Philosopher's Stone, and the know-how on how to use it. And as we find out in HBP, the Stone can be used to create a new, living body... as that was Voldemort's plan in stealing it.

And with the ability to create a living body, Albus could attempt to do something never done before in magical history... bring back the dead.

We know from HBP (and DH) that Dumbledore's biggest regret is that he led to the death of his sister, Ariana. We know he blames himself for destroying his family. And we know he believes that "no spell can bring back the dead".

He says that a lot, and if anyone would have the ability and desire to try it'd be him. But he knows it's impossible. Because he tried it.

See Voldy could come back because his soul was still hanging around, since it was tethered to the physical world. And what we learn from GOF (and COS?) is that if your soul is still hanging around, all you need to do to come back to life is build a new body. Voldy even hints to Harry in PS that the Stone could be used to bring back his parents...

So, with Albus' easy access to being able to create a new body, all he would need is a piece of Ariana's stone that hadn't moved on.

And there are two potential sources:

1. Ariana's Obscurus

Since we know (or at least highly suspect) Ariana of being an obscurial, her obscurus would have been deeply tied to her soul. And as we find out in FBWTFT, an obscurus can be preserved even if the witch/wizard has died. And with the deep tie between witches and their obscurus, a part of Ariana's soul may be preservable along with it.

2) Albus' Soul

In DH, when Harry brings back James, Lily, Sirius and Remus, he finds out that the ghostly figures that the Resurrection Stone brings back are not pulled back from the dead, but instead the parts of them that live on within Harry. As we have always been told, the ones we love never truly leave us. And this means that a piece of Ariana lives on within Albus.

So with the Stone, a few fragments of his sister, and a dash of Dumbledore's own particular genius, Albus could have (out of desperation and longing) brewed himself a Frankensteinian younger sibling. A brand new younger brother, Credence Barebone.

TLDR; Credence was the result of Albus attempting to reincarnate his departed sister.