Help me locate this teenage ghost book. Please!!

[princesslh]

"I'm looking for a book about a girl who's a great artist and grew up with an imaginary friend, whom she learns is real."
A girl who grows up with an imaginary friend. She always talked to him and she was always drawing with him. As she becomes a teenager in high school she meets a boy and falls for him. The boy is a year or two older than the girl and it turns out he was adopted. The mother seems to hate him and doesn't want him around her daughter. She tries to forbid her daughter from seeing him. The daughter can't understand why; it turns out that the mother gave up a son before she had her daughter and she was afraid that the boyfriend was that son. She never told her husband or daughter about giving a son up for adoption. The daughter began seeing her imaginary friend again and she even draws the Guy. The Guy seems to be getting weaker. It turns out that the imaginary friend she has grown up with was actually her big brother (the one given up for adoption). She meditates or something and finds out more info about this guy and it turns out he's at the hospital and has some kind of cancer.