Is harry potter related to sirius black?

According to the family tree on Wiki they might be fourth cousins once removed. James and Sirius might share great great great grandparents in Phineus Black & Ursula Flint if James is the son of Dorea Black and Chalus Potter which would make James and Sirius fourth cousins. But this isn't mentioned in the book, apart from the hint that all pure bloods are related somehow so I think Sirius is just James' friend and Harry's godfather.