The Ending *SPOILERS*
So many people seem to have interpretations of the ending, and I have yet to see the most obvious one. So hopefully this will explain things.
Sarah hated her mother so she concocted a plan to be rid of her. She faked her death in the ocean. No-one saw her go in and drown, and she left a shoe there to make it look like she had drown. No-one in the search party was ever going to find her.
Sarah then "haunted" her mother whenever she was alone, by calling to her and placing props such as her red cardigan. She knew how to open Ebrill's box at the end because she was messing around with the keys at the start and had kept the key. This is also how she knew the story of Ebrill and developed her plan.
The "shepherd" was Dave. He had abused his daughter Ebrill and performed the trepanation on her. It had left her in the state she was in, petrified and confused. This is why she killed Dave in the Abbatoir. It was also because she had just found a replacement father in James.
Everyone immediately assumed Ebrill to be some sinister incarnation of evil because it is the stereotype for bland and empty looking little girls ever since The Shining. She was only ever scared and despondent. She actually did nothing sinister and we saw this more than ever in the hospital. She then ran off after her tearful exchange with James to kill her real father, Dave.
By the end Adele was so delusional, a delusion added to by the newspaper clippings that were completely unrelated to the real Ebrill, that she jumped off the cliff because she thought it would bring Sarah back. Her and Ebrill, the innocent victim, died there and then. The "supernatural" aspects to the story are merely her delusions, and the post-jumping scenes are the last thoughts of her delusional mind before she drowns.
The ending is therefore simple. The coldness of Sarah in the car is because she is showing us her real intentions all along. She wasn't "possessed" as most of you would put it, she wanted her mother dead.
This film was set up to make people grasp for the supernatural explanation before the rational one, and they did it extremely well.