Ahah excellent question !!!
I'll need to investigate to give a proper answer.
Be sure that there will be more ninja world in the future, so chances are high that she'll be back .

I remember starting to write this world in march or april... then got too inspired by missions that were to happen in the school . In addition to the laboratory, this world was really too long. So we took out the school story, replaced it by the investigation, and... that's it .
I was so proud of the school scenes dammit . But you'll see them soon in the shinigami world (a school were Shinigami learn "special techniques" if you see what I mean ).
Shinigami world is likely to be released in november. We'll see it's not fully written yet.

I'll tell you soon about why we chose one sister over the other .