The Spider Catcher by A.L. Tyler

Bright and shy, Ember Gillespie was sent away from her home and her family when she was little; her sister Thalia, always her mother's favorite, got to stay. She spent years earning the right to come back, and to make up for a transgression she never understood.

When she finally gets home at the age of fifteen, the realization that her family doesn't want her there is nearly more than she can bear. Her grandmother has become senile, her mother is manic, and her sister is a brainwashed shadow of who she used to be. The only thing more shocking than their rejection is the reason they want her leave--Ember's mother believes that the town they live in is inhabited by demons, and that Ember is too weak to fend for herself.

Ember knows her mother is crazy. But as she gets to know some of the locals, she also begins to believe she might be right.