One Subtle Detail Reveals That Harry Potter and Sirius Black Were Blood-Related
Even though not for long, Harry found his true family in his Godfather, Sirius Black; however, they were family on more than one level as the two were actually related by blood.
Growing up without a proper family, young Harry Potter had to go through years of abuse and neglect living with the Dursleys. Even after entering the Wizarding World when he turned eleven, Harry still had no one he could call family, and the closest thing he had were his friends: Ron, Hermione, and Hagrid.
It wasn't until his third year at Hogwarts that Harry learned that he had a Godfather: Sirius Black, his parents' best friend and a known criminal who sold James and Lily out to Lord Voldemort. At least, that's what Harry was told at the time — and he was happy to discover that Sirius was not, in fact, a vile Death Eater.
Immediately after learning the truth, poor Harry wanted to move in with Black even despite knowing him for mere minutes: the Dursleys were too much for him. Surprised and happy, Sirius agreed to take Harry in, but since he was still hunted, they never moved in, and Sirius remained Harry's distant yet loved Godfather.
Except, he was not just a Godfather, and the two most likely blood relatives. This has little to do with the known statement that all Pureblood families are related in some way: there was an exact detail in Harry Potter that suggested that Blacks and Potters were related. We need to reference The Order of Phoenix to prove it to you.
In the scene where Sirius shows Harry the Black family tree, we can notice that the names and portraits of "blood traitors" were burned off it, and that's exactly what Sirius wanted to show his Godson. However, the Black family tree also revealed one peculiar marriage: Dorea Black and Charlus Potter were a married couple!
Since it was known that Dorea Black lived from 1920 to 1977, it's safe to assume that Dorea and Charlus were James Potter's parents, hence Harry Potter's grandparents whose marriage tied the two Pureblood families together back then!
This would mean that Harry and Sirius were actual blood relatives, though rather distant, and when Harry was thinking about how he found (and lost) his last real family in his Godfather, he was right on more than one level.