This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / A Tale of Two Sisters
The scene in which it is revealed thatSu-yeon is actually dead. Moo-hyeon's agonised tone of voice when yelling the truth to Su-mi, the latter's stunned and horrified reaction, and Su-yeon's horrified, broken screams as she slowly backs away from her sister, the latter desperately reaching out to her with a "no, no, this can't be true" expression on her face.
The ending flashback sequence. Su-yeon is left to die, broken and helpless, with the corpse of her mother for company. There's also the very last shot of the film - Su-mi wanders off into the woods, by herself, but not before taking a glance back at the house, as if she knows something has happened but can't quite imagine what, and the audience knows what's going to come next for the poor girl.