Just finished reading the book a few minutes ago...It is entertaining but not a 'must' read.
The 'thriller' plot factor was well done. The characters are very detailed throughout the narrative. However, using the present/past flash backs technique as a character and plot development tool is a good insightful perspective until it becomes repetitious (rehashing the same information differently without adding anything new). Unfortunately, halfway down the book that back and forth gets distracting and it begins taking away from the build-up of the moment. Ultimately, it weakens the psychological 'thriller' game of 'who've done it'.
As for the 'grand finale' between the two sisters Olivia and Jules, it feels rushed... The author (drops the ball) leaves the reader in limbo about wether Olivia ever reveals to her sister the core of her traumatic secret which is the detonating force of her mental issues and inner conflicts with herself, family and friends throughout the book.