@Background Pony  
That part was actually explained. Celestia had it in Canterlot, and kept it around in the hopes that Sunset Shimmer would return. This tells us that Celestia knew where Sunset Shimmer was and how to get there.
 
This just raises so many questions, almost none of which are answered.

@eddwardh  
Something that has been confusing me for a while: if the mirror is in the Crystal Empire, how the fuck did shimmer get in?
 
I might just have missed that bit, somehow, but that just is bothering me something fierce…

Better question, if Twilight and Sunset are both the same age, and Twilight became Celestia’s student when she was a filly (imediately after earning her cutiemark), then how old was Sunset when she ran off? And wouldn’t she have been young enough that any selfish behavior could be considered understandable? And wouldn’t she have been young enough that Celestia would have actually looked for her on the other side of the mirror?
 
This post is a lot like Equestria Girls: Poorly planned and it makes no sense.

Or Human!Shimmer lives in some other country/city in Human land.

My headcanon is the psycho killed her human counterpart.

@Background Pony  
That… Would only cement the fact Celestia care little to none about her.

@Background Pony  
I…. could accept that.

I like to think she found her double and ran her through the mirror to dispose of her, leaving the original human living some sort of life in Equestria (and an accident with unicorn magic keeping her from detection by Celestia), perhaps taking Sunset’s place with her equine family.

My initial thought was, how did she adapted into the human world at first?