Haven's Legacy — Melissa Benoist; Jessica Benoist

Haven's Legacy is the thrilling and heartfelt second book in The Powers, a sweeping middle-grade fantasy series from Supergirl's Melissa Benoist and her sister, the writer Jessica Benoist.
 
Following the events at Haven house, Ellie and Parker are prepared for almost anything . . .
 
The Power sisters have only just come to terms with the betrayal and truths revealed at Haven when their mysterious aunt, Sadie, shows up on their doorstep. Even more shocking, Sadie has a new mission for the girls: to rescue their beloved mother, who vanished years ago and is -- according to Sadie -- very much alive.
 
Parker and Ellie join Aunt Sadie at Mountain Harbor, home to the Sentry, an underground society of people with powers like their own. First call of duty: to develop their magical abilities alongside their peers. But Ellie isn't convinced everyone at Mountain Harbor can be trusted, and to make matters worse the Danger -- a smoky, tentacled monster born of greed and chaos -- has broken free of the forests surrounding Haven and followed them to the Sentry headquarters.
 
When the future of the Sentry is called into question and the Danger grows ever more hostile, the twins aren't sure who to trust. Can Ellie and Parker band together to preserve the future of the Sentry and reunite with their mother? Or will the Danger seize control once and for all, putting the future of the planet in jeopardy?
 
The Powers series:
Haven's Secret (#1)
Haven's Legacy (#2)