Harry Potter and the Order of the Phoenix
We see almost nothing of Narcissa Malfoy, Lucius's wife and Draco's mother, in Book 5. But we do discover something interesting about her: she is Sirius's cousin. Her maiden name was Narcissa Black. In fact, all of the pureblood wizarding families seem to be related to each other, which makes sense since they refuse to marry Muggles and so don't get much fresh blood in their family lines.