You Are Here by Jennifer E. Smith

She'd grown used to her family underestimating her. It was like this with everything. Emma had never even been allowed so much as a hamster, because her parents always insisted she was too irresponsible to care for a pet. Instead she'd had to make do with a series of seemingly suicidal goldfish, whose rapid succession of deaths did little to help her case in lobbying for a puppy.
Emma has one of those brilliant, abnormal, eccentric families. Unfortunately for Emma, she's too normal to fit with her family, but too abnormal to fit with anyone else. So she's learned to stand back, and watch. To feign interest. She's always felt left out of her family, and she figures it's normal (after all, all her siblings are mostly in their 30s).
Until she goes up to the attic to find her dad a book.
Hidden in a box filled with photographs, is another box containing a birth certificate for "Thomas Quinn Healy", dated the same day as Emma's, and a death certificate dated two days later, also for Thomas.
Now that Emma knows that she has a twin- or had a twin-, she's inexplicably determined to visit him. Visit his grave.
So she steals her brother's car, and sets off on an epic road trip from New York to North Carolina.
At least, it's epic until the car breaks down. She turns to the only person who she can- Peter, her map-and-civil-war obsessed next door neighbor, who's hiding secrets of his own.
Peter, Emma, and a stray dog that they pick up (or maybe the dog picked them up) set off on a road trip that's not only a trip of geography, but a trip of emotions.
"I'm sorry; I just-"
"And taking that car!"
"That wasn't exactly me," Emma began, but was interrupted again.
"Excuse me?" Mom said. "You think Patrick's car marched itself out of New York City on it's own?"
"Oh that car," she said dully. "I meant the other one-"
"Don't even get me started on that,"
This was a fantastical book.
According to my sister Valerie, it sounds "incredibly boring," but let me assure you. It is far from that. The plot might be fairly basic, and the characters weren't as original as some I have read about, but the emotion the author puts into this book- and the random facts (did you know that "okay" originated during the Civil War?)- made this book come to life. I'd give it four stars.
Peter had never kissed a girl before, and he had great admiration for those who did it so casually. To him it seemed a feat more difficult than jumping out of an airplane, or sailing around the world.