Stalemate by Angie Martin

Stalemate: a situation in which further action or progress by opposing or competing parties seems impossible.
Following her mother’s death, Calista rises as the new queen for the Arelia Region of Merfolk. She quickly realizes the problems her mother left behind: a failing region with suffering subjects. Beyond her borders, humans and merfolk kill each other at an alarming rate, but the queen, over all merfolk, doesn’t care what happens to any region, least of all, Calista’s.
With her sub-mate by her side, Calista knows she must rescue her region, but she can’t do anything in her current position. Determined to restore order to the kingdom and provide safety for all merfolk, her only hope is to challenge the current queen for her throne on Silent Night.