As far as I remember, the parentage theory on FE's part has to do with something of a dynasty; the world was different before it was put, by Hyne, into WOTC, which is the state the world is (according to him) in the game. This isn't a full explanation, but it basically says that they were changed and their memories were altered so that they'd all think they were born near the same time.

More or less. More information is in his threads, but that's enough to cover the mechanics, I think. (This isn't a perfect explanation, but it should do. I left a few things out. It's late, and my fingers are cold.)

I have to disagree about the characters not being human, because Ifrit and the Brothers specifically state the the characters are human: 'For me to lose to a human...' for instance.

As to the bond between the posse, I agree that it's closer than that of most friends, right there into family--but I see no reason to believe a biological relation. They're too different in body type, pigmentation, and...

Well. Seifer, canon, is an orphan. Fuujin and Raijin are his age, more or less. It doesn't work.