He would have had to kill someone to make her a Horcrux, the way he had to kill Tom Riddle to make the locket a horcrux. I think Harry is Gryffindor's heir. He likes making Horcruxes out of items attached to the founding fathers of all the houses. And Harry's parents lived in Godric Gryffindor's Hollow when they were found by Voldemort. I think Voldemort made Harry a horcrux shortly after killing Lilly Potter, just as he did w/his own father and the Syltherin locket. We never hear much of James Potter's parentage - although we know all of Lilly's. We only know that James is a pureblood. Hmmm.
If Harry is in fact a horcrux, he will need to destroy himself in order to destroy Voldemort.
I forgot about the killing part. I tend to jump to strange conclusions when reading (or listening to) the books. Don't get me started on what I thought about Harry having Lily's eyes.
You're right - Harry is most likely a Horcrux and will have to die before Voldemort can be finally killed. We all know Harry won't survive the final book anyway, so it's no big surprise. However, Harry will have to die before Voldemort can be killed, so it will be interesting to see how that plays out.
As far as Lily's family, we really
don't know all that much. Granted, it's more than James's family, but there's not much to go on.
I've also been wondering about the Ravenclaw relic. Has anyone been mentioned to be related to her family? Kind of hard to figure where Harry will start out looking for that one.