Here's my guess.

She fell in love with the detective, and her sister murdered her mother, and the detective knew this.
THClord Reviewed by THClord on . Can you solve this small riddle? Well... this riddle is actually a test, given to employess of a certain plant. The riddle: Once upon a time, there was an old woman, who had two daughters who lived with her. One day, this woman died. During her funeral, her older daughter met a man and fell in love for him. One or two weeks later, she killed her sister. Why does she did it? When someone post the right answer, i will say what it mean. Rating: 5