Who is Cassandra O'Koren from Dreamland and what is their importance?

Cassandra O'Koren is Caitlin's older sister. She runs away from home on Caitlin's sixteenth birthday. Caitlin looks up to her older sister and believes she is perfect at everything she takes on. Cass, however, is actually struggling herself and hates the pressure of living up to the expectations and demands of others.