A PSYCH TEST

I got this forwarded message from jay which is quite interesting. I shared this to all my close buddies and only one of them got the right answer.

A story about a girl:
While at the funeral of her own mother, she 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 and then. A few days later the girl killed her own sister!

Question: What is her motive in killing her sister?

HMMMM,,,would you know?