Narcissa is the youngest daughter of Cygnus and Druella Black. She has two sisters, though she only acknowledges one: Bellatrix and Andromeda. After Andromeda married a Muggle by the name of Ted Tonks, Narcissa was forced to turn her back on her sister - though, if truth be told, she has never forgotten her. Unlike Bellatrix, Narcissa hasn't forgotten her heart. She has merely hidden it under a guise of responsibility and duty.
During her stint at Hogwarts, Narcissa was in Slytherin House. She graduated with full marks and she could have gone on to do anything she wanted. However, because of the war climate, she chose instead to devote herself to her husband-to-be, Lucius Malfoy. They are to be married soon.