DE Sidious has the experience he had as his ROTS version, combined with a decade of extensive training, a youthful body, and multiple Jedi and Sith holocrons. ROTS Sidious gave Yoda a run for his money, so combine this with his newfound training - he would defeat Yoda as well in saber combat, I'd guarentee it.
Regarding Luke and Leia. Yes, they stopped Palpatine's Force Storm. But surprisingly, they never achieved the feat a second time, and both engaged Palpatine on multiple occasions and barely escaped with their lives. They did not overpower the Force Storm, either. They temporarily managed to combine their powers and bloodline into cutting the Emperor off from the Force for a few seconds. That few seconds was enough for the Storm to backfire, kill him, and destroy the Eclipse.