The Secrets of Dumbledore are ultimately disclosed in Fantastic Beasts and Where to Find Them. Aberforth’s estranged son, Credence/Aurelius Dumbledore, is never mentioned by the Dumbledores in Harry Potter for a sinister cause. Albus Dumbledore gathers an unusual squad to halt Gellert Grindelwald’s elevation to Supreme Mugwump of the International Confederation of Wizards in the third Fantastic Beasts film. But, most crucially, Fantastic Beasts 3 delves into the backstories of Dumbledore and Grindelwald, as well as the Dumbledore family’s numerous mysteries, with Credence at the forefront.
According to Albus, Aberforth fell in love with a girl from Godric’s Hollow in the summer of 1899, and she was soon pregnant with their kid. Aberforth never got to be a parent since she was transported away. To Newt, Albus confesses his regret: “Aberforth may have confided in me if I had been a better friend to him if I had been a better brother. Maybe things might have turned out differently. This young man might have been a member of our family.” Until the events of Fantastic Beasts 3, when Albus begs him to reconcile with his son, Aberforth keeps Aurelius a secret. But, as a consequence of his Obscurus curse, Credence is dying, and it’s highly probable that Aberforth will only get to spend a little period of time with his son. The reason Credence isn’t addressed in the Harry Potter books is because of this.
Neither Dumbledore mentions Credence in Harry Potter because they both feel guilty for not finding him sooner. Fantastic Beasts 3 shows that both brothers have a history of “secrets and lies,” something Aberforth criticizes his older brother for learning from their family. But it isn’t just Albus: Aberforth buries his memories and refuses to confront the reality of having a son who needs him. Perhaps he already feels ashamed for not reaching out earlier, or perhaps he is so ridden with guilt and grief over Ariana’s and Kendra’s deaths, that he’s trying to avoid another heartbreak. Losing Aurelius would be too painful for Aberforth to bring him up ever again, and Albus has been known to discuss very few personal matters with the Harry Potter characters. It thus makes sense that Credence is never mentioned in Harry Potter: he is the Dumbledores’ guilty secret.
There is another discussion point here: as Dumbledore confirms at the end of Fantastic Beasts 3 that Credence doesn’t have long to live, it’s safe to assume Aberforth will simply spend very little time with his son. When Harry meets Aberforth in Harry Potter and The Deathly Hallows: Part 2, they spend little time delving into the Dumbledore family’s past. Aberforth is visibly bitter towards his brother and mentions Ariana was one of the “things” Albus sacrificed in his pursuit of power. But Aberforth only says this because Hermione (Emma Watson) presses him to. It’s thus unlikely Aberforth would open up about Credence without being prompted to. Since Harry doesn’t know Aberforth had a son and Albus doesn’t find it relevant for Harry in the context of the Second Wizarding War, it makes sense neither brother mentions Credence in the Harry Potter series.
It’s unclear how much longer Credence will live after the events of Fantastic Beasts: The Secrets of Dumbledore. Albus says Credence can’t be saved, but he doesn’t say his life can’t be prolonged — in Fantastic Beasts: The Crimes of Grindelwald, he says, “An Obscurus grows in the absence of love,” and Credence now has a loving family. However, his story is bound to have a tragic end, as no one survives an Obscurus curse, and his loss will lead Aberforth and Albus to bury the subject forever. Credence’s absence from Harry Potter only stands as proof of the “secrets and lies” that have become a way of life for the Dumbledore brothers.