As far as we know, Sotha Sil only learned how to break their oath to Nerevar. Almalexia may have plotted, but it may have been a situation where Vivec did it so that the other two would go through with breaking the oath; it's always easier when the guy who told you not to do it is dead.
:D Indeed. Its reasonable to assume that Alma and Sotha went along with Vivic seeing as how Nerevar was already dead, and maybe they thought the oath was void perhaps.
Although, wouldn't Sotha Sil already be guilty of breaking his oath just for discovering how to use the tools? Or was that not technically breaking his oath?