Psychology Test

This is a genuine psychological test, believe it or not. Don't cheat.

While at the funeral of her own mother, a girl met this guy whom she did not know. She thought this guy was amazing, so much her dream guy she believed him to be, that she fell in love with him there but never asked for his number and could not find him.

A few days later the girl killed her own sister.

Question: What is her motive in killing her own sister?

Give this some thought for a while before you hit the "show answer" button. Don't cheat and be as honest as possible.