This is not a horror or a mystery movie. This is a drama disguised as a drama/horror mystery. And that's why it fails.
Swinton is amazing as always. It's unreal how convincing she is playing both the mother and the daughter. I mean, i was moved by "their" relationship, these were adorable characters. I really felt their feelings for "each other" and that is kinda insane because the viewer forgets that they are the same person. Hence the 5 stars.
However, this is not a good movie. I didn't mind the slow pace, but this movie has the most unnecessary twist of all time. Unnecessary, illogical and, dare i say, conniving. Because there are some scenes that contradict this particular twist, during this movie. Most of the people see it coming from the beginning of the movie, but then, the director convince the viewer that they got it wrong. And in the end, the thing that most of the people suspected, happened. Without any logical or paranormal explanation. Thus, as a viewer i feel cheated and i can't rate it higher.
This would be a much better movie as an art drama. No twists, no surprises. Just a mother and a daughter, their relationship and their feelings. I would enjoy this movie. You have Tilda Swinton in your movie, you don't need twists and turns. Go full drama, realistic as it gets. Psychological drama-mystery horror is another genre, much more complicated than it seems.