“Music on stage was always our playground, always our candy field, always where we were the happiest,” says Bob Cowsill, sitting on a sofa in a Louisville, Kentucky dressing-room trailer with his sister Susan and brother Paul. “Even if everything around us might not have been happy at all,” he adds. Yes, those Cowsills. “The Rain, the Park and Other Things.” “We Can Fly.” “Indian Lake.” The frisky hit version of the title song from Hair, all huge hits in the latter 1960s. They were darlings of 16magazine and Tiger Beat. They appeared on every major (and minor) variety show, including The Ed Sullivan Show. And, of course, as a group of young siblings (brothers Bob, Paul, Barry, Bill and John, with Susan joining when she was just seven) plus mom Barbara, they were the inspiration for their T...