What happens in the summer on Cousins Beach stays there. The idyllic setting for Jenny Han’s young adult trilogy, turned into the “The Summer I Turned Pretty” series now streaming on Prime Video, is at once an escape and the keeper of teenage secrets. It’s where four kids — Isabel “Belly” Conklin (Lola Trung) and her older brother Steven (Sean Kaufman) and brothers Conrad (Christopher Briney) and Jeremiah (Gavin Casalegno) Fisher — have grown up under the watchful eyes of their mothers, college best friends who won’t let go. “At the end of the day, they have an escape from the real world,” Kau…
