The Winter Duke by Claire Eliza Bartlett

When Ekata’s brother is finally named heir, there will be nothing to keep her at home in Kylma Above with her murderous family. Not her books or science experiments, not her family’s icy castle atop a frozen lake, not even the tantalizingly close Kylma Below, a mesmerizing underwater kingdom that provides her family with magic. But just as escape is within reach, her parents and twelve siblings fall under a strange sleeping sickness.

In the space of a single night, Ekata inherits the title of duke, her brother’s warrior bride, and ever-encroaching challengers from without—and within—her own ministry. Nothing has prepared Ekata for diplomacy, for war, for love…or for a crown she has never wanted.