One of the common tactics in writing, make a character look smart by making other characters stupid. Honestly, what happened to the MC's previous character development and resolve? Is like the character development just took a 360. I bet this story will be a 360 plot/character development all the way until the end.
And what happened to the authorities? Like a few chapters ago, it seems like they will playing a part but whoosh, they're out of the picture for whatever reasons. How can they let someone like this loose? I bet if a psychopath claims to have a memory lost, no one in the right mind would left them unchecked.
"I'll kill your sister." Yeah good luck with that. If there's still any logic in this series, just call the police. Who do you think they're gonna believe? A girl with a track record of mental issues and juvie records or a girl with a sister who is the victim of the former girl?