I have a theory that the those 2 siblings were killed. In the last chapter it all but says that Kairu injured Emiki. This implies the siblings can at least injurge each other if not kill each other. My thoery is that Kairu, Emiki, or Kaine killed the other 2 siblings in trhe distant past. What ever the case is with the other 2 siblings, im curious as to why Kairu fought Emiki.