Who are the sacred 28 pureblood families in Harry Potter?
I have found a list for you :) (from Pottermore!)
Abbott – There was an Abbot in Harry’s year. Her name was Hannah Abbot and she was in Hufflepuff, she fought it the battle of Hogwarts on Harry’s side.
Avery – Mentioned by surname only in the Harry Potter books. He was mentioned as a Death Eater.
Black – Sirius Black, Narcissa Malfoy (nee Black), Bellatrix Lestrange (nee Black), and Andromeda Tonks (nee Black). These are all members of the Black family, some good, some not-so-good.
Bulstrode – Again. there was a student in Harry’s year with this name. Millicent Bulstrode and she was in Slytherin.
Burke – Caractacus Burke, one of the founders of the sinister wizarding shop Borgin and Burkes, was the only member of this family mentioned.
Carrow – Amycus and Alecto Carrow were a brother-and-sister Death Eater double-act, recruited to instill some discipline at Hogwarts.
Crouch – Bartemius Crouch and Barty Crouch Jr. are the members of the family mentioned.
Fawley -Neville's mother Alice Longbottom was a Fawley, hence Neville's pureblood status.
Flint –Marcus Flint, was the Slytherin Quidditch captain in Harry’s time at Hogwarts.
Gaunt – Marvolo, Morfin, and Voldemort’s mother Merope where all Gaunts. Making Voldemort also one.
Greengrass – Daphne Greengrass (who is in Harry’s year) and Astoria Greengrass (who didn’t appear until the epilogue).
Lestrange – Bellatrix and her husband Rodolphus were among Voldemort’s most loyal followers along with Rodolphus’s brother Rabastan Lestrange.
Longbottom – Neville Longbottom was one of Harry, Ron, and Hermione’s best friends and a fellow Gryffindor in Harry’s year.
Macmillan – Ernie Macmillian was a Hufflepuff in Harry’s year.
Malfoy – Draco Malfoy, Lucius Malfoy, and Narcissa Malfoy are some of the Malfoys mentioned in the books.
Nott – Theodore Nott was a Slytherin in Harry’s year.
Ollivander – Ollivander is the wandmaker in Harry Potter.
Parkinson – Another Slytherin in Harry’s year called Pansy Parkinson.
Prewett – Harry learns that brothers Gideon and Fabian Prewett ‘fought like heroes’ but were famously killed by Death Eaters in the First Wizarding War. Molly Weasley was a Prewett, as we learned in Book 7 that Gideon and Fabian were her brothers.
Rosier - Evan Rosier was a Death Eater killed the year before Voldemort’s first downfall.
Rowle - Another Death Eater, Thorfinn Rowle is described as big and blond and got into trouble with Voldemort when Harry, Ron, and Hermione escaped his clutches in central London in Harry Potter and the Deathly Hallows.
Selwyn – Dolores Umbridge claims at one point to be related to the Selwyn family, although whether this is true or not is unclear.
Shacklebolt – Kingsley Shacklebolt was one of the wizarding world’s finest Aurors and a member of the Order of the Phoenix.
Shafiq – No appearances are made by the Shafiq family in the Potter stories.
Slughorn – Potions master and former Head of Slytherin house.
Travers – One of the Death Eaters to escape from Azkaban, Travers was famous for helping murder the McKinnons.
Weasley – Bill, Charlie, Percy, Fred, George, Ron, and Ginny are all Weasleys. Along with Molly and Arthur are the ones mentioned in the books.
Yaxley – The only Yaxley in the Potter books is another dastardly Death Eater.
This information is all from Pottermore (credit to JKR).