Since I cant do spoilers yet, Im just warning there might be some below.
I was thinking...I've been thinking this for awhile now; it's just a theory.
It's been mentioned several times, or implied, that Subaru has given Seishirou some of his vampire blood, right? The reason, yet to be exposed. But wouldnt that mean that Sei would be a vampire himself? Cause it appears if you take in the blood of a vampire, you become one yourself. And Fuuma also said that Sei appears to be aging more slowly nowadays. and vampires age slowly...
Unless, I also think, that maybe some sort of price was paid that prevented him from becoming a vampire. But I dunno...like I said, this is all hypothetical.
I cant wait till CLAMP reveals the true deal with Kamui, Subaru and Sei soon. >_<