Personally, I think Celestia always knew that Cadence was destined to be the leader of the Crystal empire and that Twilight was destined to help her, even before Twilight earned her CM. Celestia probably had been educating Cadence to be an effective leader ever since fillyhood so she wouldn't become a selfish snob like Blueblood or the other Canterlot elite. She made sure that Cadence learned humility and friendship by living and working like a normal citizen. By allowing her to foal sit Twilight, Cadence would learn compassion and responsibility while also building a strong bond with the pony that would aid her in reviving her kingdom.