As pointed out, Sirius Black was actually Draco's first cousin once removed - not his uncle. That being said, according to J. K. Rowling, yes, Harry Potter and Draco Malfoy are related - albeit distantly - by marriage, if not by blood. The Malfoys are related to the Weasleys through the Narcissa and her descent from the House of Black. We know that Ron and Draco are fifth cousins once removed. If Weasley is Sirius' second cousin once removed and if Sirius had had a child, that child would be the Weasley children's third-cousin, once removed. Through Hermione's marriage to Ron, and through Harry's marriage to Ginny, Hermione became his sister-in-law. She was also an aunt to his children and the godmother of his oldest son, James Sirius Potter. Harry also had a niece, Rose Weasley and a nephew, Hugo Weasley, through Hermione's marriage to Ron. Which mean all of them are related... Tom Marvolo Riddle, aka Moldy Voldy is indeed related by blood to Harry Potter, although it is a very distant relation. Remember the three Peverell brothers whom death gives each of the Hallows? ... Ignotus, being the younger brother to Cadmus, makes Tom Riddle and Harry Potter distant relatives. Voldemort and Dumbledore were descendants. Since both Harry Potter and Tom Riddle were descended from the Peverell brothers (original owners of the Deathly Hallows), it's true that the two mortal enemies were in fact related to one another – albeit very distantly. ... As was made clear in Deathly Hallows, Peverell blood would run through many wizarding families.

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