The story is great, no doubts, I haven't gotten to the end yet, I think I got stopped either at Chapter 8 or 9's ending, but I just gotta say one thing I learned recently.
It would be Queen Nis's thirdborn, whenever the peri princess is introduced, unless Nis happens to be plural. The grammar dictator in me won't let me move on until I mention this.
I don't know where you got that from, that's not a rule. Queen Nis' is also correct. See Merriam-Webster ;)
My English class. I was told the only exceptions are for historical figures, as Merriam-Webster confirms in the classical figures part. Perdue Owl backs me up on that front (see here). I guess it's really just up to interpretation based on your source though, so fair play with Merriam-Webster. Keep up the good work with the writing, boss!