Actually there is no evidence that the Rebels know anything about Palpatine being a Sith (except for Luke).
Although it is very possible that they might considering all the discussion between Yoda, Kenobi, and Bail Organa before they go their seperate ways. Bail knows that Yoda failed to defeat/capture the emperor and no doubt the events were passed to him by Yoda. Since he was a founding member of the Rebellion...
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