Motive Riddles

A woman killed her own sister. During the interrogation, she told a story that she just attended her own mother's funeral few days before the crime took place. While at the funeral, she said that 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 just 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. Although this woman has confessed to the crime, police are still intriguing by the story, especially because she won't tell them her motive. Hearing this tragic story, with his psychological education background, Detective Anderson easily guessed the woman's motive. Why the woman killed her own sister?