And it's always possible (hell, we're talking the Lucas universe here) that the Emperor knew Padmé survived her husband's attack but never that she gave birth to twins. He might have believed that her baby (singular) died with her. The Dark Side's a very powerful thing, but come on. Vader was standing just a few inches from his own daughter on multiple occasions and never realized it.
It and its practicioners obviously have their flaws and limitations.