Robb has a secret that he cannot tell his younger brothers and sisters. For example, he once had a sister who doted on him to the point of being lawless.
But in fact, Sansa knew that she had a sister. She often looked at her tenderly and sang hymns in a low voice on nights when she was afraid of the dark.
Arya once asked if she had a black-haired sister besides Sansa, the old woman. But when she saw her mother Catelyn's eyes that suddenly became quiet and sad, she didn't dare to tell her. In fact, when she was very young, her sister played with her. But when she grew up, she could no longer see her sister.
Bran and Rickon also have their own little secrets as brothers, such as hiding that they have a sister that no one else can see.