One of the most damning facts about the Sidereals' involvement in the Usurpation was their lack of it: they did nothing. They did nothing to prevent the attempt at wholesale murder of the Solar ruling caste by their footmen despite the fact that they saw it looming and were well aware of the consequences. They did nothing to inform the Solars of the earth-shattering treason that was about to be attempted or protect their Solar masters. As a result the Sidereals complicity in the Usurpation is a perfectly preserved secret. No one suspects the Sidereals' foreknowledge of the incident and their lac of effort to prevent it: all blame being aimed a the Dragon Blooded who single handedly perpetrated the outrage. In true Sidereal style they played their hand without anyone realising they were doing anything.
Some Sidereals had misgivings, another reason the Five Maidens didn't entrust them with active involvement. The Dragon Blooded were far lower in the chain of power and made better conspirators. A handful of Viziers made active attempts to prevent the Usurpation but they were heavily outnumbered.
Only once the full weight of the Usurpation made itself felt the reality of what they had let happen sank in and a major 'schism' of Sidereals broke off and declared themselves beyond the Five Maidens' authority. Naturally this is a heinous crime and conflict between the two sides has never ended.
The initial ambush and subsequent campaign to eradicate the Solars was only the first but most important step of the Dragon Blooded's purge of the Solar's rule. Everyone who had alliegance to the Solars had to be deposed. The other Celestial Exalted would have soon followed. The Sidereals themselves, long-standing advisors to the despised and now defeated Solars would have found themselves the target of a plan of extermination just as ruthless and thorough. Instead they hid their very existence from Creation to avoid the Dragon Blooded's revenge. Using false identities crafted with their mastery of Astrology they infiltrated Dragon Blooded society and slowly removed all evidence that there were such things as Sidereals in order to remove suspicion.
