What you see here is a theory about Twilight’s relation to Cadence. Twi’s origin isn’t discussed, she just banished from the place she came from. She is found and taken to the Castle and where she befriends Cadence. Then, they die…  
After 1200 years Cadence returns to life as a filly, she seeks out her family and stays there. She rebirths Twilight and gives her to the Sparkles, convincing them that she’s theirs, and takes on foalsitter resposibilities.