Oh my god, that was amazing. I never thought about the possibility that Yuuko slipped the Book of Clow into their house. And I love how you portrayed her and Clow, they're such a tragic couple. My favorite part of this, though, has to be how you slipped Sakura's invincible spell into the last line. Bravo!
It's a wonderful story. I like how you've managed to piece certain events together, like Yukito's appearance into Sakura's family, and a hint of how Yuuko became an alcoholic. What I especially like is that you used Sakura's catchphrase, "Zettai daijoubu dayo." That was a nice touch. Great story, faving this.