Sylvari dream and orders, question about lore

I am a new player and I wander about sylvari that join the Order of wispers. If you have secret knowlege about a topic the new sylvari would dream about it. So my question is "How sylvari keeps secrets from sylvari who dream and is likely to wake up with very dangerous knowledge. "(ending of season 2)

I think you have it mixed up, the Dreams Sylvaris refers to is "Dream of Dreams" is a state of conscious before they are awakened into Tyria.

It is during Dream of Dreams where multiple Sylvari may 'receive' the same dream from the Mother Tree

The Dream of Dreams acts sort of like a bind will to the Mother Tree, and those severed themselves from the teaching inside the dream became the Nightmare Courts

While it isn't said anywhere, one could infer that a Whispers Sylvari would undergo the meditation practices of the Soundless, cutting themselves off from the Dream of Dreams. This could also explain

The Dream also doesn't catch all information and memories- usually just the emotionally charged ones. Likewise, what any given sapling sees is a randomized jumble. The odds of anything sensitive getting leaked to anyone besides the Pale Tree is very remote.

@Musaroxy.2874 said:

Even far away from the Pale Tree, it doesn't seem right to me that so many Dream-protected Sylvari would have turned so fast.

 

Also is probable the Pale Tree itself was somehow intervening the Dream to avoid it communicating such heavy secrets. A super huge mistake, IMO, but understandable for that character at that moment.

well, according to the book, the matter of secrets came up and Killeen said, that the dream isn't very specific at all. 'a place, a face might stay with us...' I think Killeen said.

The dream does give a sylvari some knowledge of the world, how to use tools, this animal is tasty (plus the exact taste), this is not. But nothing like 'X did Y on the Z of last month that might imply that Queen Jennah is an infertile lesbian'.

So there is simply no need to cut one off of the dream.