Sperry has before been told of a brother she has/had, but her father continually denies he ever had a son.

Sperry's uncle always told her the story was true (because Ambodian wizards normally believe stories like that), and this caused her to believe it. Her sister appears to know no more about it than she does, but Arianna affirms the falsity of the story as well.

Assuming Nilrem is correct, the mystery of her father's and sister's lies remains...