They have this thing called blood bond. Once a vampire bit another vampire, they'll create a link between them. The bond gets stronger once they consume each other's. Ai already drank Ren's blood and vice versa. They will be craving for each other as nothing can break the link. Except if their memories got erased.
Zero mentioned before that he felt depraved when he was separated from Yuki. Yuki also drank from him so the bond was created between them. That's what the bond does when he didn't drink from her.
While Zero also drank from Kaname before but the latter didn't bite him so the bond was only halved on Zero's part. Or so I believe lol
Zero didn't suffer from Kaname because he has Yuki to survive. This is what I thought. I might be wrong though
Maybe bcs they are vampire and the morality in humanity between them is so thin
It's still fuckin disgusting lol
Why can’t they be normal siblings? Do they have to declare undying love in a vague way each and every time? This manga has given everything but a normal sibling relationship