Twilight cast a spell and got herself turned into a filly. Luna has tricked Celestia into believing there is no cure, and now as Luna tries to 'find a cure' Celestia has to look after Twilight the Alicorn; filly version. And heaven knows Alicorns are not born often for a reason.

Traducción Española: https://www.deviantart.com/spaniard-kiwi/art/Potras-alicornio-de-alto-mantenimiento-y-tu-990270118

As a filly living in Canterlot Castle, Twilight Sparkle either wakes up from a scary nightmare, as well as missing her family, or a thunderstorm, Princess Celestia comforts her. Just because she was her mentor didn't mean she couldn't offer a motherly touch.

Luna discovers something hidden from Celestia for twenty years, now, with emotions running high, she enlists the help of the Mane 6... but what seemed to be a single mystery, soon reveals a darker and much more complicated plot that will soon decide the fate of Equestria.

Check out the Empty Room TV Tropes Page (Warning: Spoilers!)

Revised chapters edits and proofing (with many thanks) by: lammy, Trevor, Nick Nack, Fifth Alicorn

Twilight breaks Celestia's doors.

But then, you'd be mad too, if your stupid brother got drunk and told you that you were adopted...