From the report that Cairo got from the Divine Soul Kong, it states that there was still a chance that his sister is alive. All the descendants of the Reviving King are either killed on the spot if they are resisting too much or were sent into the Soaring Dragon Kingdom to be publicly executed
The Reviving King earned too much of the Soaring Dragon Kingdom's fury. As a result, all of their descendants were treated harshly. Aside from the ones living inside the Divine Soul Kingdom, the ones outside are all killed ... or at least the ones that are discovered
The undiscovered ones are pretty low though and if they were discovered, death would be their fate
Years ago, Cairo was forcefully sent into the Tower of Ambition while his sister remained in the Larton Kingdom. Kira can join him to enter the tower and keep living if she wants but she didn't do that
