SPOILERS
This film had two strikes against it for me as a viewer. 1) I am officially bored to tears with "shocking" "Shalamanesque" endings where the narrative we have been following for a good 90 minutes turns out to be a fabrication from an 'unreliable' narrarator/filmmaker. 2) I had just watched the 1972 film The Other the day prior which has essentially the same plot twist.
The switcheroo of an ending... SPOILER
The one sister, Su-yeon, is dead and the surviving sister, Su-mi, was not only imagining 1)that her sister was still alive but 2)she, Su-mi, was also acting as the "wicked stepmother" in these staged (and often violent) dramas as some form of catharsis.
...did not elicit the appropriate response from me and did not seem as logical as say the switcheroo ending to Shalaman's Sixth Sense or Fincher's Fight Club. It seemed to me this tact was taken mainly to shoehorn in some shocking scenes of violence and eerie supernatural moments.
Now, having said that, I really liked this film whcih is unusual for me because a shoddy twist like that is usually a deal breaker. However, what saves this film is that the film doesn't end with the twist. The actual ending to this movie, the final scene, is legitimately haunting and I don't mean that in the "spooky" sense. It turns out it is a heartbreaking story being told in this film, one of deep regret and loss, and the trio of women who head the cast are superb. They really sell it, especially the two sisters Su-mi (Im Su-Jeong) and Su-yeon (Mun Geun-yeong). The humanity that they bring to their performances along with some rich, atmospheric cinematography and memorable music truly elevate what could have been another staid genre excerise into something much more.