The mother
Are we supposed to believe that the mother had been dead for a while, and that's why the father wouldn't put the heating on, so she wouldn't start rotting? Because I was left with that impression. The house was a mess and looked like it hadn't been cleaned in a while, and it would sit well with the father's behaviour.
Sorry if it was explained somewhere along the movie, as I felt it was very slow at times and didn't pay much attention in some places. Great performances by the actors, though, that's what kept me watching.