Intruder

Let's start off by saying this is almost certainly the best movie ever made containing a character named Bum-Suk.

A brother ignores Mother's warning to hold onto his little sister's hand when they go to the playground. The sister vanishes without a trace. 25 years later, the brother is a father himself, and a widower, his wife having been killed by a hit-and-run driver. A mysterious woman appears, claiming to be his long-lost sister, but he has his doubts, even with the DNA match.

Sister tries everything she can to convince him, but when he catches her in a lie, his suspicion grows, even as his family blindly accepts her story. She even tries seducing him, but to no avail. He knows something is wrong.

This film leaves a lot of loose threads and the motivation is sketchy, at best, but it is entertaining.