For the Throne by Hannah Whitten

 

"Red and the Wolf have finally contained the threat of the Five Kings, but at a steep cost. Red's beloved sister - Neve, the First Daughter - is lost in the Shadowlands. But Neve has an ally, even if it's one she'd rather never speak to again - the rogue king Solmir. Together they must journey across a dangerous landscape to find the mysterious Heart Tree - and finally claim the gods' dark, twisted powers for themselves."

For the Throne continues (and concludes) the story of twin sisters, Red and Neve, both in different ways locked into inescapable destinies. Red is now happily settled in the Wilderwood with her Wolf, but Neve is trapped  in the upside down, shadowy monochrome world of the Shadowlands. Neve must journey through this horrific landscape to find the enigmatic Heart Tree, which might somehow, with luck, and a huge dose of sisterly love, show her the way to return to the world above. On the way there are monsters and gods to contend with, and no one to help Neve - unless she can be persuaded to place her trust in her enemy Solmir.

Most of the story takes places in the creepy world of the Shadowlands, following Neve and Solmir - and what a scary, disturbing world it is! Fortunately there are brief respites, and flashes of colour, as Red works from her side of the barrier to reach Neve. 

There is perhaps a lot of trudging along through the Shadowlands' wastes, but at the same time this gives Neve time to reflect on some of her earlier decisions, and on Solmir's character and intentions - for the first,  acknowledging that they may not have been as right as she believed, for the latter - well - that he may not be as out and out evil as she'd assumed. In both regards, Neve has a lot of personal growing to do.

All in all a gripping mix of magic, horror, and sisterly love.

I haven't read what might be considered the first half of this duology - For the Wolf which follows Red's story - but enough references are made to the events in it to fill in the background while still leaving the reader wanting to know more.