Okay, Sirius can't be back alive. I think that he's dead, and part of what Harry needs to learn is that he's going to loose people. That's part of life. Maybe since Sirius died for him, he will learn more about love, and its powers. That could help in defeating Voldemort.

Also, Harry DOES need a family, but I think that the Weasely's do a great job. Snape doesn't really owe him anything, but if he did, it would probably end with the "Someone-Knows-That-Harry-Needs-To-Live-And-Will-Therfore-Die-So-He-Can-Live" theory. Maybe Snape will know the importance of Harry's survival.

Snape will (should) have a bigger part in the coming books.