I think that Cadence was born a royal Crystal Pony over 1,000 years ago, but was sent out by her royal pony parents because they knew the rule of King Somber would ruin her life and take her rightful throne (they themselves probably already being dethroned and forced to live amongst the common Crystal ponies). So, like Oedipus, she was sent out of the Kingdom so that she could grow up in a better life. AND, because apparently all the princess ponies can live seventy billion forevers, she didn't get very old while she grew up as an Alicorn in nearby Canterlot. I imagine she was adopted by somebody in Celestia's court because she was an Alicorn and Alicorns are all royals. She'd grow up and her special talent would be being the queen of the Crystal Kingdom, which is why her cutie mark is the crystal heart, but she wouldn't understand it meant that until the Crystal Kingdom reappeared. Celestia understood the mark but kept it a secret from Cadence that it was what her cutie mark meant until the kingdom came back. That's why Cadence can create love, too, because she was meant to power the Crystal Heart along with the other ponies of the Crystal Kingdom.