SaI'kapəθi

I ran into this really cool test the other day on the internet that was used by a famous American Psychologist to test if a person has a mentality of a psychotic killer. Read on and leave your answer/justification as a comment. I will post the recommended answer and then we can all learn a little about ourselves.

Scenario:
" A woman, while at the funeral of her own mother, met a guy whom she did not know. She thought this guy was amazing. She believed him to be her dream guy so much, that she fell in love with him right there, but never asked for his number and could not find him. A few days later, she killed her sister."

Question:
What is her motive for killing her sister?