Harry Potter would therefore be a blood relation to the Black family through the female line. This may help explain the close relationship between Harry and Sirius Black. In the Order of the Phoenix, Sirius remembers that when he ran away from home as a teenager, he was given a place to stay by Harry’s grandparents and they treated him as their own son.
In the Mirror of Erised, Harry’s grandfather is the one with the knobbly knees. If it is true that James Potter was the natural son of Charlus and Dorea Potter, that would make them Harry’s paternal grandparents. It would also mean that Harry was a second cousin to the Black sisters, Bellatrix Lestrange, Andromeda Tonks and Narcissa Malfoy. It would also mean that Draco Malfoy, the hated school contemporary of Harry Potter was a second cousin, once removed.
It is entirely possible, within the realms of the fictional characters as told by JK Rowling, that Harry Potter would also be a third cousin to his great friend Ron Weasley and his sister Ginny. Further delving into the newest Harry Potter book, Harry Potter and the Deathly Hallows, may reveal more of his complex genealogy.