The FF do like Jo and admire things about her. EB seems to feel something and have charity. Can't quite reconcile this with being cruel to her own kids - unless there is exaggeration in that claim.
But Jo, however much her uncanny direction finding, skill at climbing and fearlessness etc is admired by the main children, is simply not viewed as as fully human as the FF by the narrator.
She's purely there as picturesque local colour, and at the end the Kirrins' cook Joanna (who shares her room with Jo when she stays overnight at Kirrin Cottage, presumably on the grounds that she couldn't possibly share with George and Anne, just as Ern had to have tea in the Trotteville/Linton kitchen with the servants) is taken in hand by Joanna who says something about having a sister or cousin who likes 'bad girls' and will give her a home. Like she's a rescue dog.
And I know a lot of novelists -- being able to write with a degree of empathy in no way precludes you from being a ghastly human being! Her daughters have gone extensively on record about their mother, and one daughter wrote a memoir. They disagree on EB's qualities as a mother, but even the well-disposed daughter admits that she was her mother's favourite and that her sister was less fortunate.
And even leaving aside her daughters' testimony and the more dubiously-founded stuff about the lesbian affair with her daughters' nanny and the naked tennis, there's a lot that is a matter of public record or in her diaries. For instance, her first husband, Hugh Pollock, an editor at her publishers, was married with children when she met him, and she vowed she would bag him in her diary. Later, after they'd married and she was having one of many affairs, with Kenneth Darrell Waters (a surgeon who would be her second husband), her husband agreed to be the wrongdoer for the purposes of the divorce if she would let him have access to their children - so as not to wreck her reputation and career -- but she never let him see his daughters again after the divorce, and succeeded in ruining his career in publishing, so he ended up bankrupt.
I think she was in many ways brilliant, and ahead of her time in self-marketing (down to the famous signature) and cultivating her child fans via fan clubs, magazines etc, but also a nasty piece of work as a human being in many ways
Can you imagine Aunt Fanny and Uncle Quentin or Aunt Allie and Bill Cunningham carrying on like that while the children were off tracking down smugglers and exploring secret passages? 