I read a book a while back that i absolutely loved only i cant remember the title or the author. Help?

It takes place during the summer when the main girl and her twin brother are introduced. They both have a best friend whose a guy and hangs out with them both. the brother ends up going to summer school and the girl and the best guy friend work at the camp or resort over the summer. The girl works as a tennis instructor and the guy does something i just cant remember. The girl likes the guy and because of this another chick hates her because she wants him too and is much more popular than she is. and to make the chick stop liking the guy the main girl and the guy pretend to date. then at the end of the novel the guy falls head over heels for the girl at a water volleyball tourn. and kisses her while she is wearing her red bikini. I really wanna read this book again even though i basically just gave you the entire plot line. I just have been looking for days and cant find the name or author of this book. But i know the front cover has a picture of two beach chairs, towels, two pairs of flip flops and a bag. I just really need help please?!?