Lock and Key by Sarah Dessen
(Description from Amazon.com)

“Ruby, where is your mother?”

Ruby knows that the game is up. For the past few months, she’s been on her own in the yellow house, managing somehow, knowing that her mother will probably never return.

That’s how she comes to live with Cora, the sister she hasn’t seen in ten years, and Cora’s husband Jamie, whose down-to-earth demeanor makes it hard for Ruby to believe he founded the most popular networking Web site around. A luxurious house, fancy private school, a new wardrobe, the promise of college and a future—it’s a dream come true. So why is Ruby such a reluctant Cinderella, wary and defensive? And why is Nate, the genial boy next door with some secrets of his own, unable to accept the help that Ruby is just learning to give?

Best-selling author Sarah Dessen explores the heart of a gutsy, complex girl dealing with unforeseen circumstances and learning to trust again.