WARNING! Contains SPOILERS for Fantastic Beasts: The Secrets of Dumbledore.
Fantastic Beasts: The Secrets of Dumbledore finally revealed Credence/Aurelius Dumbledore is Aberforth's estranged son, and there's a dark reason behind the Dumbledores never mentioning him in Harry Potter. The third Fantastic Beasts movie sees Albus Dumbledore rally up an eclectic team and stop Gellert Grindelwald's ascension to Supreme Mugwump of the International Confederation of Wizards. But most importantly, Fantastic Beasts 3 explores Dumbledore's and Grindelwald's backstory and the many secrets of the Dumbledore family, Credence being at the forefront.
In the summer of 1899, as Albus explains, Aberforth fell in love with a girl from Godric's Hollow, and she was soon pregnant with their child. But she was sent away, and Aberforth never got to be a father. Albus expresses his regret to Newt: "Had I been a better friend to Aberforth, had I been a better brother, he might have confided in me. Perhaps things would have been different. This boy could have been part of our lives, part of our family." Aberforth keeps Aurelius a secret until the events of Fantastic Beasts 3 when Albus urges him to reconnect with his son. But Credence is dying as a result of his Obscurus curse, and it's very likely Aberforth will only get to spend just a short amount of precious time with his son. This is the root of Credence not being mentioned in the Harry Potter series.
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.
