Two weeks later, she killed her sister.
This question was reportedly asked to a bunch of serial killers who all gave the same answer.
Now, us psychs are good, but not THAT good that we can pick serial killers by just asking one question. You would need batteries of tests to pick the psychopaths.
Nevertheless, why don't you play along and tell me why you think the woman killed her sister. And I'll tell you if you have the potential to be a serial killer.