I had so much fun with this because it has SO MUCH PASTEL COLORS anyways remember this is just my interpretation and it's not canon~ i know Cadance has a canon backstory, but i just did my own MLP Princess Cadence's Secret (Rebirth) inspo credit! :D
1) The first royal family of the Crystal Empire, Emperor Amadeus Venus, his husband Scultura di Cristallo, and their daughter princess Amore Bellisima.( Royal Family Tree-NG btw the reason Scultura di Cristallo isn't here it's because i didn't know with who i would ship Amadeus lol). Cristallo is what is know as a "Twinkle eyed", they were the artisans of crystals, and it is said that their eyes were like that due to exposure to them. After many years, Amadeus and Cristallo dies, Amore is crowned Empress and Sombra attacks her, transforming her into a crystal statue, killing her. The Crystal Heart was an artifact made directly connected to the soul of the Empress and her subjects, so it stayed dormant until Amore's soul found another body.
2) 1000 years later, a little baby named Mi Amore Cadenza was born in Heart and Hooves day to three mares,Canzone Estiva (yellow pegasus), Bridal Beauty (Pale pink pegasus) and Rosetta (Bright pink earth pony), they named her like that because of a tiny heart birthmark she had near her eye. The three mare were wedding planners, so growing up Cadance saw all kinds of love in their store. Little Cadance often had nightmares with a shadow figure turning her into crystal, and woke up with screams of "Save them! Help the crystal ponies"
3)12 years later, in a Heart and Hooves days, a heartbroken pegasus prepared what he thought it was a love potion for his loved one, and poured it into a cloud, making it rain above them,it could have been easily fixed…if the cloud didn't turned into a storm cloud, raining all over Canterlot. Chaos soon started, fiances running from their loved ones, mares and stallions leaving their work just to see into eachother eyes, even Celestia was affected! Starting a relationship with a…piece of cake. It seemed that everything was doomed, if it wasn't for Cadance, since she stayed at home. She tried to make them reason, because that wasn't true love! You can't make somebody fall in love by force! It hurted her heart…it was then when from her heart a bright light glowed, taking all of the love in Canterlot, Cadance set every little heart filled with love in their respective place, love always finds a way. After that she was surrounded with the same bright light, and the next thing she knew she had a horn.
4) Queen Celestia was so grateful with her, and she admired that a little filly understood so much about feelings and ascended all by herself. Cadance was crowned Princess Mi Amore Cadenza. She stayed with her mothers until she turned 18, when she started her princess training. She told Celestia (who she considers her aunt) all about the nightmares, and she recognized it all immediately, when the Crystal Empire curse had vanished, she would return and take her place as the Crystal Empress.
Btw the reason Cadance mothers weren't the ones planning their own daughter's wedding, it was because being such good planners and the mothers of a princess, the business skyrocketed and they were very VERY busy, so they let their daughter's future sister-in-law and her friends take care of it.