A terrific debut. 4/5 stars. 
 This review was originally posted on my book blog. 
 This review will be rather sparse as I think the less you know about the plot of The Sister, the more you'll enjoy it. And the more I say, the more spoilery things will get, so I'll keep this brief. 
 I read The Sister in one, 4-hour sitting and I think that's probably how it's best enjoyed. The plot switches between events in the past and present, keeping us in the dark and building suspense while slowly revealing the characters' secrets. I thought this time jumping was well-handled and it only confused me once (I think because it had been alternating past and present and then there were two consecutive chapters in the "now"). As long as you pay attention to the "Then" and "Now" markers at the start of each chapter you'll be fine. 
 My only frustration was with the main character who I dearly wanted to smack hard more than once. I do understand that her gullibility is essential to the plot - if she kicked the person being nasty to her in the bum after the first suspicious thing they did then the story would be over about a third of the way in - but it didn't stop it being rather frustrating at points. 
 However, that aside, this is a well-paced story with plenty of twists and turns to keep you guessing. There are enough revelations and hints to allow you to make some guesses before the final reveal (and get some of them right!), but enough is held back to guarantee a few final shocks. 
 Overall: a fantastic debut and I look forward to reading Louise's next book!