A Head Full of Ghosts by Paul Tremblay

So, this novel is a psychological thriller in which a teen girl appears to be possessed very much like the girl in the exorcist movie and so her family, because the father has been out of work for over a year, and for other reasons, decides to be on a reality tv show based on the girl's possession/psychological problems, which is to culminate in the exorcism of her "demons".  The story is told by Merry, the younger sister of the possessed teen, Marjorie, in a series of interviews and blog entries given 15 years later.  Merry had been 8 years old at the time of the incident and we see the whole thing through both her 8 year old point of view and her older self's point of view.  The story is captivating and contains plot twists and surprises.