Hidden wives / Claire Avery.

Enhanced descriptions from Syndetics:

Fifteen-year-old Sara and her beautiful sister, Rachel, are too young to legally drive a car--but are approaching spinsterhood in Utah's secret polygamist Blood of the Lamb community. Having long since reached the "age of preparedness," they will soon be married off to much older men chosen by the hidden sect's revered Prophet.

As Sara, chosen to become her uncle's fifth wife, grows more distraught over her impending incestuous marriage, she begins to scrutinize the faith she has followed blindly her entire life. But for Rachel, who will be married to one of the many powerful community leaders vying for her hand, disobeying the Prophet means eternal damnation. Her friendship with the newest member of the community, the young and handsome Luke, starts as an attempt to save his agnostic soul, but ends with the pair falling helplessly in love. When Rachel is forbidden to see him, her absolute faith in the Prophet is severely tested.

When Rachel's future husband is finally announced, violence erupts, and the girls must find the strength to escape the only life they have ever know...before it's too late.

Claire Avery has woven a stunning tale that could be ripped from today's headlines. Shocking and empowering, Hidden Wives is a page-turning debut that will stay with the reader.