The gathering (shadow house, book 1)

Poppy Caldwell Is An Orphan Who Keeps Seeing A Figure Standing Behind Her In The Mirror At The Group Home Where She Lives, But She Does Not Associate This Ghost With The Letter That Mysteriously Appears In Her File, Claiming To Be From A Long-lost Aunt, And Inviting Her To Larkspur House–and She Is Just One Of Five Children Who Find Themselves Gathered In This Strange House With A Deadly Past, And Apparently No Intention Of Letting The Children Escape.