I was just about to post this exact topic. Anyway my answer:
The girl's sister knew how to contact the guy because she had his sunglasses, and with that she could see the store and ask about the guy who bought them. However, the sister wouldn't give the sunglasses to main character girl because she wanted to sit on them. Main character girl told her she has a week to give her the glasses or else she would kill her. The sister refuses.
So on the first day, the main character asks her sister to give her the glasses. The sister refuses as she wants to sit on them. So main character girl (lets call her MCG) goes to the store and buys a single boot.
The next day, MCG asks her sister to give her the glasses or else she'll kill her at the end of the week. The sister refuses as she wants to sit on them. So MCG goes to the store and buys a rubber band.
The next day, MCG again asks her sister to give her the glasses or else she'll kill her at the end of the week. The sister refuses as she wants to sit on them. So MCG goes to the store and buys a small knife.
The next next day, MCG again again asks her sister to give her the glasses or else she'll kill her at the end of the week. The sister refuses as she wants to sit on them. So MCG goes to the store and buys a leg exercising machine thing.
MCG asks her sister for the last time and she still refuses as she wants to sit on the sunglasses. So, MCG gets the leg exercise machine and works on one leg for 3 days. That leg becomes super buff! She takes the boot and attaches the knife on the end with the rubber band and puts the boot on. She goes to her sister and kicks her in the face, killing her. She finds the sunglasses but they have already been sat on. MCG is now sad...
Okay, I just made that up on the spot in case your wondering...