From reading the book that doesn't surprise me. Neither does it from the perspective of what I understand about familial abuse.
The need for a shroud of secrecy is great, whether perpetrator or innocent family member. To be part of a happy, normal family is a strong desire to the extent that any threat to that may be covered up, smothered and denied: However true.
I cannot see any particular reason for her making it all up. Indeed the story is not of the cruellest abuse imagineable which in itself lends the book an element of believeability.
The narrator was strong and brave throughout and remains so for telling her story as part of her healing process. If the story is true then the denial is typical cowardice on her mother's part.