How could Yoda know about the Sith?

At Qui-Gon’s funeral Yoda mentions there can be only two Sith: a master and an apprentice. But the “rule of two” was established by Darth Bane, who survived the Sith wipeout without the Jedi knowing about it, and this new order was meant to keep it that way. Bane’s idea succeeded, for we hear earlier in Episode I that the Sith are actually considered long extinct by the Jedi. Then how could Yoda have known about the rule of two?

I never thought about that before, good question... I think it's probably just to do with the fact that the EU isn't officially connected to the films. GL probably originally invisioned that there were always only two of the Sith. Then some EU guy came along and decided that it was a little dumb for there have been only two of them in the entire history of the Sith order. (can't say I disagree) So he wrote the thing about Darth Bane establishing the "Rule of Two" and established what the Sith are now. I bet if you asked GL he'd say that he didn't come up with Darth Bane and that the EU doesn't have any connection to him. Just a guess.

dunno, but Bane is mentioned in the epI novel, but don't have that one

i think insider said that the jedi had recovered some of bane's writings

Yup, nvm the Bane thing, the only official bit from Lucas is that this is a rule the Sith eventually adopted- no mention of that being some huge secret.

Yeah If GL has a vision he'll ignore EU

In E1 when Mace was spouting about the Sith, Yoda remained silent....as though he could have added more but did not wish to reveal his knowledge at that time....hmmmmhg!

guess he was worried about what the Siths are capeable of doing

it's ... magic 😉