I would agree that Fanny does not cast aspersions on Maria and Julia because she knows it would do no good. She wouldln't be beleived... and she would only be "being cheeky" to her benefactor, who would then take the opportunity to remind her that she was only a poor relation that he'd taken in out of charity and that she had no rihgt to speak so about people like her cousins.
However I think taht also Fanny (a) would not "tell tales" anyway unless absolutely pushed ot it... and (B) is still so conscious of the "superiority" of hte Bertrams that she would nto take it on herself to criticise them....