Anima by Marie Buchanan

To all appearances the Mintons seemed to be an ordinary family living in the English countryside. There was a maiden aunt Olive and her cousins Edward and Megan. Nothing of significance happened to any of them–until the Blanchards moved next door.

Perhaps it was the house. Perhaps it was the attraction Olive felt for Giles Blanchard. Whatever it was Olive suddenly became aware of a second Self. Someone else, a stranger, was now living in her body, speaking her thoughts, planning terrifying deeds…