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Yes, Harry Potter and Voldemort are related, as read in Book 7, Harry Potter and the Deathly Hallows. In the story written by Beedle the Bard, three brothers created three Deathly Hallows in order to become immortal. The Elder Wand, the Cloak of Invisibility and the Resurrection Stone were created.

 The brothers turn out to be the real Peverell brothers, and the Deathly Hallows were passed to their descendants as they passed away, except for the Elder Wand, which was passed on to whomever defeated the previous owner.  However, the Resurrection Stone is passed onto Voldemort via his grandfather (Marvolo Gaunt), whom he kills, while Harry receives the Cloak of Invisibility via Dumbledore from his father (James Potter), effectively proving that Harry and Voldemort are distantly related through the Peverells.

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