I thought she did find out that this guy was actually her sister, after they had a great time together.
Her lost sister who had a sex transformation, appears on their mom's funeral. They both fall in love with each other, have sex and stuff but soon they find out that they are actually "sisters".
Angry about the fact that her dream guy is her own sister, she kills her.
Hmm, I thought the girl and the guy got together.
But the guy cheated on the girl with her sister, at the first opportunity. That's why the girl killed her sister.
What does this tell you about me?
Edit: I think the various ideas people have are actually the most interesting part of this thread.
I remember that Veus said the man was the murderers "Dream" guy...so my guess was that she killed her sister to see if it were all real or not. Wait...come to think of it, that is not a very good guess, because the murder took place days after the funeral...dammit.