Somewhere in the middle of Haunted (1995), Robert Mariell kissed his sister Christina in the painting room while David was spying on them. Why did he kiss his own sister? What is the significance of that scene?
It is to show how messed up the family are. The originally present as a normal, healthy family with a slightly batty grandmother.
As we get to know them better we realise that they are seriously messed up. The implied incest is an example of this.