YA novel about a teenage girl's younger cousin that goes missing

[Taylor]

I remember reading this book sometime in 2010/2011 - pretty sure it was published around that time, or perhaps a few years before. It was about a teenage girl whose younger cousin stays with her family for the summer. Right off the bat, she grows irritated by him and his unwavering positivity/whimsical personality. Can't remember too many details, but he ends up going missing, and she has to find the person responsible for his disappearance. I vaguely remember the cover having yellow accents and an image of an adolescent boy.

[Taylor]

SPOILER




Upon discovering fishing line in her boyfriend's bedroom, the girl comes to the conclusion he's the one who kidnapped/killed her cousin. I believe they find him in a body of water in their neighborhood.

[Kate]

Taylor ~~
You can "bump" your thread every month or so. This pushes your thread back to the top of the folder instead of languishing here on page 213 from seven months ago where fewer eyes will see it. Get back in the game! Bump!
You can do this by typing a new comment at the end of the thread, or even by typing the word "bump."
Good luck!

[Rainbowheart]

Absolute Brightness

[Cecilia]

PLOT DETAILS & SPOILERS
I am looking for the same book!!
I think I remember the book cover being blue & yellow. She also finds a necklace or something of his under her boyfriends bed. Then once hes charged, her boyfriend tells her that her cousin had actually given that to him. &an old lady finds his shoe in a lake and that's how they discover his body. I think the cousins name in the book is Louie? I'm not completely sure.

[Rainbowheart]

This book is Absolute Brightness.

The cousin's name is Leonard.

Hope OP comes back to check.