Yes...Narcissa is a Sirius's cousin along with Bellatrix and Tonks' mother, they are all sisters.
I get the feeling that Voldie ol' boy has not told THAT many people that he is not "pure blood". He has issues... I mean it is a known fact that he is the last descendent of Salazar Slytherin a known pure blood advocate, he was in Slytherin House, his father left his mother-witch because of that fact, so that is whe he choose Harry as his equal since Harry is a "Half-Blood" himself (which I don't get, because both his parents were magical...I guess they are going off a Lily since she grew up Muggle...anyway) and not Neville who comes from a very strong Magical Family background.
I really think only a few people know that Voldie is not pure-blood like Dumbledore and Harry....if I remember the last scene of between Harry, Draco, Crabbe and Goyle...Harry's mentions that Voldie is not pure blood, it's implied that Draco, Crabbe and Goyle did not believe him and were attempting to jinx/hex him before McGonagall came through the door.
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