Bainbridge’s novel Young Adolf extremely closely - and openly - follows the Liverpool portion of The Memoirs of Bridget Hitler (Duckworth, 1979, ed. by Michael Unger), the book mentioned by Cecil. He’s correct in saying that the Memoirs were quickly discredited by historians, notably in this NYRB review (payment required) by Hugh Trevor-Roper. While Cecil obviously tends towards the theory that Bridget (the usual spelling of her name) had a hand in the memoirs, this is far from obvious. For instance, as Trevor-Roper and others have pointed out, the text is far too well-written to be the unaided work of someone who was, by all other accounts, near-illiterate. At the very least, some sort of ghost writer must have been involved, in which case it’s rather surprising that they’ve never broken cover. Of course, the same might be said of any independent hoaxer.
Bainbridge’s novel actually came out the year before the published edition of the Memoirs, but this was partly a result of her persuading her publishers to then issue the original. She’d necessarily added what she had to as a novelist, but she manages to include virtually all the Liverpudlian details touched on in the Memoirs. The novel’s a better read, at that.
Incidentally, the supposedly startling - and hopelessly implausible - revelation in the Memoirs is not the story of the Liverpool visit that they’re usually remembered for, it’s the claim that it was common knowledge in the family that Hitler had personally shot his neice Geli.