Jess/Sara theory- spoilers
While I don't believe the theory saying they were one person is right... I was thinking about something else.
I was thinking was that it was Sara who actually saw the parents' corpses, not Jess.
She somehow managed to repress all those memories and hide them away deep inside her to the point where she believed it was her sister who witnessed the scene. Jess was the one living with guilt that she did not share Sara's experience as they grew a bit distant. She was not as great coping with parents' death and tried to commit suicide before.
That would however explain why Michi said he did not want Sara to go in the forest- because of Sara's 'sadness' which was the reason why the forest was attracting her and feeding on her fear. The spirits clearly did not want Jess- otherwise she would be long dead.
It was only in the cabin where Sara saw the vision resurface- but I was almost thinking- she covered the little girl's eyes, she KNEW what was in the basement, she asked her to cover her eyes before she even got to the bottom of the stairs, so she must have been aware of it subconsciously what was really there.
In the beginning there was nothing, and it exploded.