How the Hero became the enemy

Christi is a short-tempered and beast-like girl. She lived happily and freely with her "brother", Cloud. Well, Cloud was the closest thing Christi had to a brother since she never had a real family. She was raised by Cloud and Tifa. And when Cloud went off to go save the world or something, she would go wether he liked it or not. But Christi had a secret: she was also part black mage. So she was a pretty good fighter. But one day she....... just disapeared....
Not many years later Cloud found her...... working with the worst of all people.... Sephoroth. Cloud tried to interigate her but she seemed to have forgotten who he was. But it was true, she was on Sephoroth's side. But not only that, she was his...... apprentice. And from that on, Cloud attempted but failed many times to try and bring back his sister. Cloud had almost lost all hope, until he finds out that Christi crossed over to a world that he's never seen before. Would that be his chance to get his sister out of that creep, Sephoroth's hands?