Could Luke and/or Leia have killed Emperor Sidious?
I think so. In TESB, Kenobi called Luke their only hope, and there was another hope (Leia), and, in ROTJ, Luke was called their only hope by Obi Wan because he had to kill Vader (and presumably, the Emperor, too, because with Anakin dead, who else could do it).
Luke Skywalker was the galaxy's only hope because he was a force user and he was the only person that could kill Darth Vader, and heavily implied, if not blatanly told to us, the Emperor too. A non force user is no match for a powerful Sith or Jedi in a fight. Therefore, only a force user could kill another force user (with some exceptions, such as the clonetroopers outnumbering the Jedi, but, Palpatine knew the rebel's tricks and he could easily escape the DS2).
Kenobi and Yoda knew that Luke was their only hope.
And, if Luke died or failed to complete his task to kill the two Sith Lords that ruled the galaxy, Leia would then have to fullfill his role, for, as anakin skywalker's daughter, she was a force user too.
With Anakin on the darkside, Luke's role was now to be the new "Chosen One", and Leia would do that if he failed. (a new hope is another word for the new Chosen One). Even wookipedia stated that yoda and kenobi depended on the twins to fullfill the prophecy
Now, things didn't go quite as planned, actually Anakin fullfilled the prophecy in the end by destroying the two sith lords (himself and sidious).
Now, suppose, here's the question. Could Luke Skywalker have done it directly, by killing the Emperor too (and not just Vader, he could kill Vader possibly, he almost did) and saving the galaxy from them? Or, even more interesting, could Leia have killed the Emperor? Or even Vader, for that matter?
I think yes. I think that if Anakin never turned away from the darkside in the original trilogy, it's very possible that Luke or Leia could have killed the Emperor, unlike Kenobi or Yoda.
Luke could force jump to dodge the Emperor's sith lightning, and then kill him with his lightsaber.
But here's the problem. How would you even compare/measure how two force users would fight against each other?
Mace Windu could only win his duel against Sidious because he used Vaapad.
What force powers did Leia have to fight the Emperor with? Could she dodge his attacks? If Luke died, Leia didn't have a freaking lightsaber to fight Palpatine with? And he would just use the Force against her. We're talking about straight up fights, not Vader sneakily killing the Emperor when he's vulrenable.
I'm not that familar with force powers, so describing a duel with force users of different powers isn't exactly too easy for me.
But Darth Vader in the OT couldn't have killed Sidious in a straight up fight (that's why he needed Luke to kill the Emperor so that they could rule the galaxy as father and son), Sidious had force lightning that would damage his suit, and with that heavy weight of a suit, Anakin couldn't run and dodge the Emperor's attacks.
If Vader needed Luke to kill Sidious for him, that means totally that Luke could have killed Palpatine in a straight up fight. If you disagree with my evidence, please explain why. Speculation for all!
But Leia is a totally different story, without a lightsaber or force training. 😛 😛 😛