Disclaimer: Faith, Buffy, and the Sunnydale crew belong to Joss Whedon and Mutant Enemy. Paige and the Charmed Ones belong to Constance Burge and Aaron Spelling. I'm just keeping myself (and hopefully others) amused. Don't sue me, hire me!
Faith Matthews had had parents die on her twice now- and both in the same way. The first set, the ones she was born to, were drunk coming home from a Fourth of July party, and wrapped the car around a tree. Faith, in the back seat, was bruised and traumatized, but essentially unharmed. So, at the tender age of six and a half, she was shipped from Boston to San Francisco to live with her mother's cousin and her husband and daughter. She and Paige, who was 'nearly ten' when they met, got along like any other sisters might, Faith tagging along with everything Paige did, to the elder's public annoyance and secret glee.
Paige began to rebel, however, and despite Faith's gratitude to the Matthews for adopting her, she followed where Big Sis led. Faith kept watch while Paige smoked her cigarettes or made out with boys, they snuck out together, dressed in the same punky, slutty clothes, even tried alcohol together. Their parents tried a hundred times, a thousand ways, to bring them back into line, but Paige was having none of it, and of course Faith followed. Until, one afternoon, on their way from Paige's school to Faith's, and caught up in the same endless argument, there was another car crash, this one leaving Paige traumatized and confused but unharmed fifteen feet from the car.
Both girls, stricken with guilt, vowed to reform, and Paige began to fight the system to get herself Emancipated Minor status and a job, so she could take over guardianship of Faith. On her sixteenth birthday, she was legally emancipated, and a week later she and Faith had an apartment paid for by the state, contingent on them both maintaining good grades.
Paige graduated near the top of her class, and only three years later had a degree in social work, helping other families like theirs. Faith still dressed and acted like a tough, punky chick, but she kept a B average, and even had a part-time job for spending money. Their lives seemed to be entirely back on track...