I'm not really surprised because I've read the characters' profile before I read the manga and watched the anime. But, even if I read the characters' profile beforehand, I still wouldn't be that surprised because Kaname and Yuuki do have a visible resemblance with each other.
And in my point of view, they're not THE "siblings" anymore since Rido sacrificed the real Kaname (real brother of Yuuki and who happens to share the same name with Kaname) to Kaname, who is the ancestor of the Kuran family, so that Rido can make him his servant.
They may be siblings biologically, but intellectually, I think they're not. Well, that's just my point of view.