OMG I just realised something. The mirror to the human world is originally shown in The Crystal Empire castle, so how tf did Sunset travel to the human world the first time using the mirror when the Crystal empire was non-existent? The mirror wouldn't have existed at that time.
Yeah but I would have thought that Celestia would have kept that mirror in her own possesion, especially after what happened with Sunset. Seem's like an oversight on Celestia's part if she let Cadence take the mirror, especially if Celestia expected Sunset to return one day.