An extremely simple but effective plan. Nobody would know what that thing really is, yet who would dare to question the emperor's word? So, they could have made a "holy relic" that will explode no matter who touches it, to discredit Sasha. Anyone with at least half a brain would understand the emperor and empress have absolutely nothing to gain from Sasha's existence, so there's no way they would go out of their way to confirm Sasha's claim to the throne. Unless their plan was a longer one, in which they would pretend to take good care of Sasha, to gain respect from more righteous people, but then later had Sasha "accidentally" die.