Paranormal suspense; starts with a boy and girl getting abducted from their family into a new one and forced into studying a weird religion. A detective follows the murder of girls.

[Bryan]

Looking for a book i read about 7-8 years ago. It starts with a boy and girl getting abducted from their family into a new one and forced into studying a weird religion they followed. The children manage to escape when they are older and start to try and have their normal lives, the boy is blinded by rage and allways alone in hes room, the girl wants to forget everything and start over. Even tries to fall in love. it has different time lines, it tells a story also about a detective following the murder of girls but never manage to caught the killer. the killings were unique cause he marked the girls and the killing was done in certain times. After a while the sister enters the boys room and discovers he still practices the religion and by the end you discover the boy is the killer but hes kinda possesed by a demon. I dont remember the ending well but i think the detective finally gets him and discovers some of the truth. Please help ive been looking for this book for years...

[Lobstergirl, au gratin]

Is this for adults?