I was looking at the Black family tree and saw that Dorea Black married a Charlus Potter. It listed them having 1 son. This could be harry's dad. Dorea and Charlus were Sirius's Great Aunt and Uncle. I'm pretty sure this would make Sirius and Harry 2nd cousins. If they are related then it explains why Harry got Sirius's house when he died, because it said in the book that only a Black can own the house. As for Harry's mother's side, I think his mother's family was related to Godric Gryffindor because Harry's mom had red hair and green eyes and so did Godric Gryffindor. This would explain why having his mother's eyes is so important. Harry could be the Heir of Gryffindor.