A Lady of Secret Devotion (Ladies of Liberty, Book 3) by Tracie Peterson

Eager to ease the financial burdens of her family, Cassandra Stover is thrilled when she is hired to be a companion to Mrs. Jameston, a wealthy society woman. Cassie finds herself living a life she'd never imagined-and growing close to the elderly woman who treats her like a daughter.
Her new position is tarnished only by Mrs. Jameston's son. As Sebastian's resentment and cruelty toward his mother mount, Cassie begins to harbor concerns for her employer's safety--and ultimately her own. When Sebastian is suspected of fraud, insurance investigator Mark Langford solicits Cassie's help to uncover the truth. But what begins as a game of pretense is quickly complicated by matters of the heart.