Slightly different from "do people know that Palpatine is a Sith", in that it seems canon that people are unaware that Palpatine is Sith Numero Uno... I'm more curious why nobody seems to have twigged?
For example - Yoda is aware that Palpatine is a Sith. Yoda is also, for some time, in contact with what will be the heads of the Rebel leadership - he had just escaped from an "exciting" Force-battle with the Emperor... but didn't tell the leadership? Wouldn't the leadership want to know this, wouldn't it be pretty good propaganda for their cause?
Palpatine also walks everywhere dressed like a crazy hobo/Sith Lord. Pre-Emperor, he's all "look at my face, here I am in ceremonial robes", and post-Emperor... well, you know. Didn't... any of the troops/people he regularly met notice this?
Everybody presumably knows Vader is a Sith Lord - in the very least, all the troops/commanders around him know this. Didn't anybody wonder why someone as powerful as Vader just took orders from the Emperor... unless maybe the Emperor is also Sith Lord? In the original trilogy we aren't explicitly told Palpatine is Sith - but we sure can work it out pretty easily.