Karaokes do have cameras inside rooms, and i'm pretty sure if someone is getting beaten up inside the guy at the reception or security is instructed to call the police. What is more, it is not so hard to pinpoint who is the culprit, even if they wear masks, there is a lot of evidence and witnesses. It is quite an easy work for the competent police, competent that is. By the way, what prevented the sister from lying that she saw her face clearly?