First and foremost, let me clear out that I really don't like Kaname as much as some other people, but the amount of cloudiness over his real relation with Yuuki makes me so angry, because it proves that you didn't read the manga! And heck, I'm not even a hardcore VK fan.
Here is what really happened to Yuuki.
As you know, Juuri and Haruka Kuran were siblings. They were the siblings, not Yuuki and Kaname. But, take into account that this is a different species (at least, in VK) that we're talking about. Purebloods usually marry their siblings to keep their bloodlines clean.
Juuri and Haruka had two children. Yuuki and her brother, who remains unnamed in the manga. Rido Kuran killed Yuuki's brother, and Yuuki was left as the only child.
As most (hopefully) of you know, Rido Kuran resurrected Kaname.
Now, here's the catch.
Kaname is the first king and founder of the Kuran bloodline, so he is probably related to Yuuki only about a hundred times removed, and that's basically as related as you and I are.
He was taken in by Juuri and Haruka to serve as an older brother to Yuuki and substitute the child that was lost.
Why do you think that Haruka's actions when Rido arrived at their house to take away Yuuki affected Kaname so much?
Even though Kaname was not Haruka's child, he protected Kaname from Rido's...bubbly...monster...thing. Whatever you want to call it, it's legitimate, since I don't know what that mess of goo was myself.
Besides, Kaname even explains to Takuma (or Ichijo, as some of you know him) that he isn't Yuuki's real brother.
When Yuuki gets her memories back (I'm sure you remember that chapter), Kaname says something along the lines of: "If only I were her real brother," to Zero. Poor prefect - he was freaked out, but back to the point here - that gives you an explanation.
I hope I made it clear.
If you don't believe me, go to Wikipedia, or even better: READ THE MANGA. Yes, again. No, it won't hurt you.
Toodles!
~DQ
P.S. You should be really grateful that I wrote this. My left hand is in a fricking cast, but I typed it up for you...actually, I didn't. I just got angry at all the people who were online and whining about incest. If that were true (I'm not talking about Juuri and Haruka - they're dead, for Pete's sake), I wouldn't have read the manga myself. So, yeah, there you go. This is an awfully long P.S., so I'll stop here.



