The Secret World of Briar Rose by Cindy Pham

One hundred years have passed since the last heir of Gyldan fell into eternal slumber and doomed the once-mighty kingdom to poverty and invasion. At least, that’s what the fairy tales claim.

Corin is a jaded thief who doesn’t believe in fables, even when she searches Gyldan’s underground tunnels to find her younger sister, Elly, who ran away to find the sleeping princess in hopes of a better life. Corin’s conviction is challenged when she discovers the ruins of the ancient castle, maintained by beings from the kingdom’s golden age, who protect a hidden portal into Princess Amelia’s subconscious. Following Elly’s voice, Corin jumps in the portal and seals the entry behind her.
Inside the lush world of Amelia’s dreams, the sisters reunite for a new adventure as they meet Briar Rose, Amelia’s whimsical alter ego, and Malicine, a sharp-tongued demon with a gift for magic. But as they explore ice castles, sunflower mazes, and star-filled oceans, Corin suspects Briar Rose is hiding darker secrets behind her “perfect” paradise—and that there are some things her subconscious can’t bury forever.