This book was an interesting concept, it follows an older sister who covers for her younger sister's killings. I read it in basically one sitting as it is pretty evenly paced. However, much of the book is simply about her sister's feelings and domestic life as the less favored sister. Very little of the thriller is actually "thrills". The MC is mostly unlikeable to me, as she seems to have no problem protecting her sister and covering up her kills until someone that she likes may be the next target. The book is also written in a rather confusing way where the chapters switch between current times and past but there is no indication of when that is happening until an age is mentioned half-way through the chapter.  The plot is interesting as it is slowly revealed additional information about their dysfunctional upbringing and why the MC feels the need to protect her sister so much. 

Where the book fails to me is there is no character development. You would assume that eventually she gets tired of protecting her serial killer sister, and tries to save a victim or out her sister or something, but that's not how it unfolds. The sister's motivations seem petty and immoral and there is no change in who she is. The character you get on page 1 is the character you have on the last page. 

Overall I enjoyed it but if you like a fast-paced thrillers with multiple reveals, this book may not be for you.