Psycho Question! HAHAHA

For all my viewers, I once got this question from a friend of mine and it is a question that tests someone if he/she is in fact, a psycho i think... Hmmm.. Here goes the question....

Once, there was this girl which seemed to be normal from anyone's point of view. One day, her mother tragically died due to heart failure. The girl was obviously devastated by her mother's death. She couldn't sleep, eat, or even walk properly in her everyday life. During her mother's funeral, she kept on crying and crying without stop but suddenly, he saw a man and she suddenly fell in love. She just couldn't stop looking at the guy. Once the funeral was over, she tried to look for the guy everywhere but he was nowhere to be seen. She was stomped. Later that night, she killed her sister unsuspectedly. The question is, WHY did she kill her sister?

I answered this question wrongly and i'm glad, because if i were to answered it correctly, i would've been a psychotic freak! HAHAHA!
If any of you guys answered it correctly, then you might have that psycho blood in you. MUAHAHAHAHA(gelak setan)

Just give me your answer and i'll tell you if its right or wrong.

Tips: The bold text are some hints i created myself to unveil the answer! HAHAHA