A Consuming Fire - Laura E. Weymouth

Anya and her twin sister Ilva grow up in Weatherell, a remote village bound by tradition to sacrifice a girl to the mountain god every eighteen years. When Ilva bravely volunteers, Anya stays behind—only to learn her sister will never return. Refusing to accept this fate for herself or anyone else, Anya sets out on a dangerous journey of her own. But unlike Ilva, she isn’t going to appease the god—she’s determined to destroy it.