Okay, First, sorry if this has been done before (or to death)
, but I couldn't find the thread for it and I only have like two minutes to get this idea down.
So, In the Hearts Warming episode at the end they hang a flag with the Two Sisters on it, which shouldn't be right because Equestria was just made and whatnot. But what if, since the three tribes were coming together, they simply elected the two alicorns to be the princesses.
Like Luna and Celestia were just two fillies that happened to be alicorns and everyone was like, "They are all three pony types make them rulers," and the flag was made then. Or I could be missing something.
Again sorry to bring up season 2 during season 5.