More of my au where Celestia adopts and raises Twilight.
 

 
With the banishment of her sister drawing to a close, Celestia had fallen deep into depression. She did her royal duties and upheld her cheery personality, but in truth she was so very unhappy. She missed her sister. She missed how things used to be. She wanted to have that personal, loving relationship again - the warmth that only came from family and someone who needed her on a level her subjects didn’t. Without Luna, she didn’t have that, and if it wasn’t the guilt eating at her it was pure loneliness.
 
Perhaps that was why, one summer night, she looked upon the foal left on the castle grounds and felt a strong tug on her heart. It wasn’t entirely uncommon for ponies to leave unwanted children near the castle. Celestia and her guards made sure the foals safely got where they needed to be, as was a little known duty to the people. This foal, however…This small, purple unicorn was somehow different. She arrived at a time Celestia was at her lowest.
 
The guards tried to reason with her. They tried to convince her to let them take the filly to a nearby orphanage, but she wouldn’t have any of it. She wrapped the foal in her wing, and firmly dismissed the guards for the night.
 
Deep down, she knew it was an irresponsible thing to do. She was constantly busy, never had a moment for herself, and the castle was no place for a little filly to grow up. But, just once, Celestia wanted to put herself first…
 

 
I imagine little Twily wants to know why she has to go to bed, while her mama gets to stay up late.
 
When you’re a princess too you can stay up however late you want, Twilight.