Vanessa Fogg’s 2016 The Lilies of Dawn is a standalone fantasy.
Kai’s mother and foremothers have served as the Dawn Mother’s priestesses for many generations. They live next to the sacred lake and never marry. Nonetheless, they have daughters. Each priestess chooses her successor from among her daughters. Traditionally the successor is the older daughter. But Kai’s mother chose Kai, the younger daughter, over her older sister Suna. Kai cannot understand the why of the odd choice; unlike Suna or their mother, Kai has never heard the Dawn Mother’s voice.
Not that it matters, since their ancient way of life is doomed.