Where is Harry's father's family? What happened to them? Sirius says he lived with them after he ran away from home (in book 5), but nothing else about them is ever mentioned that I can remember. Does anyone have any theories? Were they possibly killed in the attempts to find James and Lily? Were they wizards? Muggles? And why does Harry never ask anyone about them?

I've often wondered about both the Potters and the Evans. How did all the rest of Harry's family (on both sides - Potter and Evans) die?

We know they're dead, as JKR said that Harry has no Living relatives - and thats about the extent of it.... we dont have any further fact, only speculation at this point.

"Now Now Draco... Play nicely"
Harry Potter and the Chamber of Secrets

the only living relatives we know are only Petunia, vernon and dudley