This was doing really well, until the final quarter. And then what the fuck.

Excuse me while I rant.

Katherine - Honestly deserved to be a better character, at some points she was cartoonishly villainous, and others just smart enough. She would have been a good queen without Natalia and the Queens riding her back.
Mirabella - Bottom line, you wasted your strongest character. Mira from Book one would be disappointed.
Arsinoe - I don't understand why she ran off to the mainland, when in the last book they talk about how much Billy's family dislikes her. They would have her sent to a mental hospital just for being her, and Billy wasn't any help when they were there the first time. I get the needing to forge an alliance but would it have been so hard to just have Billy stay an extra couple months and not leave AS SOON as it's over? Then he and Arsinoe could have traveled back to the mainland together and you could have some scene referencing back to when they wanted to travel, and them talking about going to each country and allying. Jules couldn't tell him they'd help him take care of his family? Billy couldn't wait a couple weeks to heal? I just don't think that was handled well.
The three in general - I would have also just loved to see a story where they live. Katherine would make a good queen, Mira didn't want to be, and neither did Arsinoe. Arsinoe could have gone back to the mainland, and Mira could have led the temple.
The mist and the dead queens - What the hell with the mist. Honestly such a disappointment with that. Such a scary, lingering, big deal, just to boil down to, it leaves when it wants too. And then just taking out the dead queens like poof! And the mist killing Katherine but not Arsinoe?? I just. Ugh. Also I really wish the rules of Low Magic had been explained better, or at least fleshed out more. At some point it just became blood magic most people just weren't educated enough to use or understand. And when did it stop costing Arsinoe anything? The first book made low magic seem like an actual risk, but now it's nothing?
Also the throw away line, 'they may stray but they always come back' about Sandrin boys. Matthew is different, but I seriously feel like the author was trying to justify cheating at this point. The Joseph plot line and him just being accepted back was so garbage and unneeded, especially with where this story went. CHEATING IS NOT OKAY. DON'T JUSTIFY IT WITH 'TRUE LOVE'.

All in all I just felt like this was going in one way and decided to take a sharp left towards the end to throw its audience for a loop. It just solidified that Book One was my favorite, even while I wished the sisters could just be together.

Also how fucked up is it that all three sisters are never actually together for a conversation/something close to friendly at all in the entire series.

Also Fuck Emilia.