technically, Twilight is part of royal family: Celestia/Luna => Cadence => Shining Armor => Twilight Sparkle you see where I'm going here? also, inb4, another idea from Friendship is Witchcraft.
Fair point. Although if Twilight was a blood-relative, that would make Cadence and Shining Armor cousins... Yeah, she's probably not blood-related.
Wouldn't she now be a princess by marriage though? They did call him Prince Shining Armor and Twilight is his brother. Now all we need is for Daenerys Targaryen to come across the sea with her dragons. Winter is coming.
If we use a real life example (I know it's a silly idea:derpe Pippa Middleton is not called Princess Pippa. So I assume that Equestria works in a similar way.
Game of Thrones reference. So is the "Winter is coming." There's lots of convoluted monarchy stuff in it.