A small army of Kennedys descended upon this ski town-turned-indie film nexus yesterday for the premiere of “Ethel,’’ documentary filmmaker Rory Kennedy’s love letter to her mother. From Ethel Kennedy herself, diminutive and cheerful at 83, to Bobby Kennedy Jr., his siblings, their spouses, cousins, and grandchildren, the clan turned the packed red carpet outside the MARC theater into a transplanted Hyannisport. Paparazzi cameras flashed hectically, whirred faster when festival founder Robert Redford arrived to chat with the film’s subject, then went into a seizure-inducing blitzkrieg when Swift turned up.