Argh! I couldn't put this book down. I kept trying and trying, but finally had to accept my life was on hold until I finished it. 
 Psychological thrillers are tough. Either they're predictable, or the ending is so unrealistic it's unbelievable. The Sister pushes toward the latter, but at least it's always interesting. I was definitely never bored. I had some issues with motivation, though. 
 Sometimes it was a little confusing what time period the book was dealing with, as it alternates time frames, but not always chapter by chapter. And as the protagonist's present circumstances really started heating up, I resented being dragged back to the past. Also, her past really seemed like it was set in the '80s, so it would be startling when a character would have a cell phone or sing a Beyonce song, etc. (Just read in another review that there were "Then" and "Now" chapter headings, and so there are! I didn't even notice them...too funny!) 
 I got a little frustrated by some of the repetition--all the vomiting, which other reviewers have mentioned--even though Grace's vomiting as an adult is eventually explained, she does it a fair amount as a child/young woman as well. And does every light have to be described as a "(insert type of fruit here) glow"? But those are small nitpicky things that a good editor could/should have caught. 
 I was also frustrated by how dense Grace was. It took her way too long to realize what was going on, but the fact that I was that up in arms over a work of fiction shows how effective Jensen is as a writer. Like I said, thrillers are tricky, but this is one of the better ones I've read. And that says a lot. I much preferred it to the ones that have had movies made and are hogging all the press lately. Unlike the other psychological suspense writers I've tried, I'll be sticking with Louise and will get her next book. 
 A solid four stars.