Naturally I've already seen the Deathly Hallows, Part I. It's hard to believe its been over three years since I finished reading that book (in two days). I spoke briefly about how genealogy has been incorporated into fantasy world literature before. However, the movie versions of these books must leave out those things. Movies are visual and based on the actions of the characters. Movies always falter a bit with long explication. So, if you are only a Harry Potter movie fan, you are missing something. [SPOILER ALERT: if you are just a movie fan, you may wish to stop reading now.]
For instance, Ron Weasley and Sirius Black are third cousins through the Black Family. Both are great-great-grandsons of Phineas Negellus Black, headmaster of Hogwarts. That also makes Ron and Draco Malfoy, third cousins, once removed. Nymphadora Tonks (Dora Tonks) is Draco's first cousin despite their being on opposite sides in the war against Voldemort. It isn't apparent in the movie that Bellatrix Lestrange and Narcissa Malfoy are sisters (the third sister being Nymphadora's mother). And thus far, it isn't clear that Harry Potter and Voldemort are cousins through the Peverel family either.
I did enjoy the fact that J.K. Rowling used medieval English family names such as Lestrange and Peverel in her world. I really do descend from both families.