Margaret Peterson Haddix's Under Their Skin

  Nick and Eryn's mom is getting remarried, and the twelve-year-old twins are skeptical when she tells them their lives won't change much. Well, yes, she says they will have to move. And they will have a new stepfather, stepbrother, and stepsister. But don't worry, Mom assures the kids. They won't ever have to meet their stepsiblings. . . .
  For Nick and Eryn, this news begins a quest to find out who these kids are- and why they're being kept hidden.
  In New York Times bestselling author Margaret Peterson Haddix's two-book series, the answers Nick and Eryn find could have reverberations far beyond one family's happiness . . . to the fate of all humanity.

  I got this book this past Christmas. I wasn't sure if I was going to enjoy it, but I decided to give it a chance.
  I finished the book by the end of Christmas day, a feat I rarely accomplish.
  This book is very well paced and full of excitement. So much happens in this book, yet I was able to keep track of what was going on.
  This book contains no swearing, violence, and nothing proactive. The story itself is well written, keeping a steady pace....until the truth comes out.
  I recommend this book for all ages!