There are many things that don't add up with Kiku. Remember that, for instance, when she made Mahiru fall in love with her the first time, she seemed exalted at the idea of being now able to "turn him into a vampire" (Don't know the english translation). This would not fit in many explanations, because she is clearly not aiming at just turning him into a vampire now.

There is also a more simple explanation: if a vampire bite a human they love, the vampire dies.

This would fit well into Miku clear obsession with death, it would explain why she is taking it so long with Mahiru (she wants an happy ending, not just any ending) and it might explain why Nazuna's mother died so mysteriously, while her father was alive and well.