Introducing Winter Star, the would be second born child of Twilight Sparkle and Princess Celestia.
 
Winnie here would have been born from Twilight but named by Celestia since Twilight got to choose the name of their son. As an alicorn, Winnie would have inherited the role as future ruler of Equestria while Sirius would pursue his passion for astronomy. In the AU in which she was born, Celestia never died and Twilight wouldn’t have adopted Hazy Sky. Due to Tia being alive, Sirius would never develop his obsession with being an alicorn and would grow up adoring his goofy baby sister.
 
Interestingly enough, Hazy and Sirius would meet in this AU but as enemies due to his pursuit of questionable magic.