Why did Gemma hide her past from her family and only reveal certain parts to Beeca and no one else?

Gemma's memory of the past has been wiped out by Nazi gassing during the Holocaust; she can only recall what happened in connection with snippets of a fairy tale.  Becca's sisters are scornful of Gemma's seemingly irrelevant tales, her mother is a shadowy, inconsequential character, and her father, although sympathetic, has a scientific mind and is not interested in unraveling the meaning behind Gemma's stories.  Gemma senses that Becca is the only one who possesses the caring, tenacity, and imagination to listen to her stories and take them seriously.  Only Becca will investigate the clues to uncover the secrets of her grandmother's life.