I completely agree. I might be a bit biased because Sirius is one of my very favorite characters, but I think that-while Sirius maybe did struggle with place he found himself in his post Azkaban life, and while, yes, he could be quite reckless-it wasn't out of lack of paternal feelings towards Harry. The "mixture of father and brother" idea I think is very spot on, but at the same time, he does have a wealthy of godfatherly and parental/paternal instincts regarding Harry: he worries about him, he goes to great risk to be near him when Harry's in trouble in GoF, he even wants to legally adopt him for heavens' sake. Can you imagine the scene that resulted when, once the Order became aware that Harry and Luna and everyone were in the Department of Mysteries with the Death Eaters, someone (maybe Remus or Molly) tried to tell Sirius that he couldn't go fight, that he couldn't help save Harry? Sirius would never have let them go to the Ministry to rescue his godson without him, never, no matter what the consequence. (Sadly, in his case, the consequence went too far.) Sirius loved Harry like a godfather loves his godchild, and I imagine-not having any godparents myself-that that might well be a mixture of predominantly paternal feelings mixed with older/younger brotherly love and companionship. Paternal feelings, however, were the dominant ones.