Looking for horror movie title?

I'm looking for a horror/psychological film. Unfortunately, I only have a rough idea of ​​the plot in my head. It's about a family living in a large estate, quite isolated from the city. During the film, I think the family members are killed by a murderer who somehow lives in the attic or something…

In the end, the brother imagines that the family is still alive and the sister lets him believe that.

Hope someone can help me.