I need to post here...I don't know if anyone has realized this yet.
"Rido stole the Kuran's first-born son"
"The Father Kuran tells Kaname right before he dies that 'He's sorry that Kaname never got to become his cute son'"
Okay..obvious now.
Rido stole their son, who is Kaname, brought him to the Kuran Ancestor's Grave, and used Kaname's body as a "shell" or a surrogate of some sort, just like Shiki is right now, to "host" the ancestor's body so he could feed off of the strongest blood, etc.
Either way, Kaname's "body" is the actual body of the Kuran's first born son, who is Yuuki's brother. But the consciousness that inhabits the body is that of the ancestor Kuran.
I hope that made sense.