Three ponies, an Earth pony, unicorn and pegasus had unloving homes. It was all they had in common. They adopted each other as sisters, believing family was about who loved you no matter what, not who you were related to.
This wasn't enough for them.
They decided to try and combine their talents to create a a spell that could make them real sisters. They didn't know how or care how. Their biggest inspiration was from a story of a unicorn who made his own friend, his own enemy and eventually the love of his life.
They created a spell, but it didn't work as planned.