Haven's Legacy (the Powers Book 2)

After sisters Parker and Ellie McFadden's mysterious aunt Sadie shows up, they join an underground society called the Sentry, where they are given a chance to hone their newly realized abilities, but they soon learn of a smoky, tentacled being, and the twins must rely on their new friends and teachers to help them defeat the fearsome threat and uncover the truth behind Sadie's mysterious assignment.