"Reading group guide"--Back cover.

Sage Winters always knew her sister was a little different even though they were identical twins. They loved the same things and shared a deep understanding, but Rosemary—awake to every emotion, easily moved to joy or tears—seemed to need more protection from the world.
Six years after Rosemary’s death from pneumonia, Sage, now sixteen, still misses her deeply. Their mother perished in a car crash, and Sage’s stepfather, Alan, resents being burdened by a responsibility he never wanted. Yet despite living as near strangers in their Staten Island apartment, Sage is stunned to discover that Alan has kept a shocking secret: Rosemary didn’t die. She was committed to Willowbrook State School and has lingered there until just a few days ago, when she went missing.
Sage knows little about Willowbrook. It’s always been a place shrouded by rumor and mystery. A place local parents threaten to send misbehaving kids. With no idea what to expect, Sage secretly sets out for Willowbrook, determined to find Rosemary. What she learns, once she steps through its doors and is mistakenly believed to be her sister, will change her life in ways she never could imagined . . .