Something strange is going on here, but what?

Overview

Siblings Elsa and Anna are temporarily sharing a house. Driven by grief, the days are torture for Elsa until one night she observes a strange phenomenon. Initially it was a doll that floated into the room, but the cases become more and more frequent. Meanwhile, Anna has trouble believing Elsa's descriptions, even though night after night chickens seem to disappear inexplicably from the coop. Something strange is going on here, but what?