A humbler, and actually probably less likely, theory about Sunset Shimmer · 8:59am May 20th, 2017
Remember the Reflections arc in the comics? Princess Celestia went back through the mirror to say a passionate and heartfelt goodbye to Good King Sombra after Starswirl found out; then she went back to say another passionate and heartfelt goodbye, and then to sob her heart out over what just happened to Luna, and then because she was so desperately lonely and had no one at all to talk to, and one thing led to another and suddenly it was a thousand years later and their two worlds were about to crash together.
What if Sunset Shimmer was their foal, abandoned on an orphanage's steps as a foal and "coincidentially" "discovered" by the Princess a few years later?
That might explain why there doesn't seem to be a native Sunset in Humania; it might also explain Celestia's mix of awkwardness and genuine warmth with Sunset: she's trying to balance her love for her secret daughter against her desire to not be too excessively nepotistic.
But then again, there's also a widespread theory that Twilight is Celestia's secret daughter, based on Celestia's similar warmth with her. I'm not quite prepared to say that both of the personal students who Celestia overloaded with hopes and dreams also just happened to be her foals; I'd rather hypothesize that this is just the kind of pony Celestia is. She's motherly, supportive, and kind, but she teaches like a wise old trickster, and once in a while she parents like a wise old trickster, too.
This sheds a lot of light on some of Sunset's other relationships; I'll post that in a moment.
(But I still think I prefer Sunset as Spitfire's sister. Similar personalities, possibility of more hilariously corrupt pony cronyism, and Firebird Dahlia is too good to pass up.)
Not the first time I've seen this proposed. Sunset at least viewing Celestia as a primary mother figure, even when her biological mother is still alive, is rather common. That said, I don't think I've seen the "interdimensional scion of two royal houses" concept before. The light and darkness motif actually works very well for Sunset. Looking forward to seeing where you go from here.
4539666
It didn't take the 2014 comic annual to convince me that Sunset had strained relations with her biological family, if she has a non-Celestia-related one. There are kinds of ponies that she always got along with pretty well, but not all of them.
And "interdimensional scion of two royal houses" is a terribly grandiose way of putting things, so it suits Sunset perfectly.