Summer Trilogy by Jenny Han

Belly has always lived for the summertime, because summer means all her favorite things: swimming, the beach, and the Fisher boys, Conrad and Jeremiah. For as long as she can remember, she’s shared her summers with the brothers at Cousins Beach. And for as long as she can remember, she’s been in love with Conrad. Then one summer it seems like he might have feelings for her too—but so does Jeremiah.
As the summer seasons pass, Belly has to choose between two brothers who love her…and she’ll have to break one of their hearts.