The betrayal / Diane Noble.

Enhanced descriptions from Syndetics:

Ensnared in a religion she increasingly questions, Bronwyn faces an impossible choice.

When recently widowed Bronwyn becomes the second wife to Gabriel MacKay, she promises her new sister wife and best friend, Mary Rose, that the marriage will remain platonic. But she finds herself falling in love with the man who has promised to keep her safe--even as her heart struggles with her promise to her friend. When Gabriel marries a third wife, and as the practice of polygamy increases and the age of brides decreases, Bronwyn begins to question the righteousness of this new religion.

Even as the Saints journey to Salt Lake Valley, their new promised land, Bronwyn and Mary Rose plot their escape. But when they are suspected of treachery by Brigham Young's secret army--whose mission it is to punish apostates--the two sister wives face the most difficult decision of their lives, one that can mean life or death for those they love.