There is a way that this can end moderately happy. Something I posted on the Bat when Mr. Creepy Self-Proclaimed Kidnapper first appeared was basically:
"Sachi pins the kidnapping on the creepy guy and has Mister Kidnapper either call the police or come to her rescue somehow. This will accomplish several things necessary for a happy end: one, it clears her Onii-chan of her kidnapping (who would believe that the self-proclaimed one is a fake after being caught with Sachi or evidence of her (she might very well plant the gratuitous Sachi photos on him somehow: updated from this last chapter), second, it gives the two of them a socially acceptable reason to become close (it would not be unnatural for a girl to crush on her rescuer/hero), third, being in the limelight will help protect Sachi from her parents and teachers and her classmates to a degree (so she would be more likely to survive until she turns eighteen or whatever the age of majority is in Japan.)