Partially inspired by SUPERKITSCH take on Amore and the princesses, here: [ NG ] holy trinity by SUPERKITSCH
—-
In the KirinDOS verse, Princess Amore Ouranios, Goddess of the Dawn, was Celestia and Luna's maternal cousin. Raised alongside them, the trio were extremely close to one another, Amore often acting as a a mediator between her cousins' conflicting personalities.
For centuries, the three alicorns maintained peace. During the Long Winter and the foundation of Equestria, Amore had discovered the Crystal Heart in the Frozen North, and used it to create an area where her ponies would be safe from the Windigoes, forming the foundations of the Crystal Empire. Amore later wielded the Elements of Harmony with her cousins in order to seal Discord in stone, bearing Kindness and Loyalty. Things were grand.
Of course, until they weren't.
No one knows what really happened the day the Crystal Empire fell. According to the accounts of the very few Crystal Ponies that escaped, a mysterious dark wizard named Sombra had taken the Heart and used it against Amore, turning her to stone, and shattered her. Celestia and Luna avenged their cousin, but not before the Crystal Empire and all of its inhabitants were lost, victims of Sombra's vindictive curse.
Amore's loss divided the two sisters, who both blamed themselves for not being able to save her. Celestia buried her grief in her work, raising herself up as the protector of ponykind, unintentionally isolating a hurting Luna, leading to feelings of resentment and bitterness that festered and later became the catalyst for Nightmare Moon.