Sister Wife by Shelley Hrdlitschka

Celeste, Nanette and Taviana live in Unity--a religious compound in which it is believed that salvation for men results from having at least three wives and salvation for girls and women depends on becoming one of those wives. Celeste is about to turn 15 and is rebelling at the thought of being given in marriage to someone as old as her father. Her younger sister, Nanette, on the other hand, can hardly bear to wait another year before she's deemed ready. And Taviana is not of Unity, but has found refuge there from a terrible life outside. When Taviana is expelled from the community and Celeste falls for a boy her own age, all three girls must choose what they are and aren't willing to sacrifice for their future happiness. For high school.