A Dark Dividing by Sarah Rayne.

This 2011 novel by Sarah Rayne is wonderfully strange and gothic.

At outset, a reporter has a mission to uncover a mystery surrounding a family. He becomes compelled by the photographs that Simone Anderson displays at the Thorne gallery. 

Somehow the journalist can detect Simone's dark secret:

"She had been four years old when she became aware of this inner darkness, and she had been a bit over five when she began to understand where it came from.

The other little girl, The unseen, unheard child whom no one else

could see or hear, but who lay coiled and invisible inside Simone's mind. Simone did not know her name so she just called her the little girl."