Lowell, its architecture, its accent, and its hairstyles circa 1990, is in many ways another costar of “The Fighter.’’ To hear the brothers’ mother and sisters go at it is to be back in a specific time in a specific place. Russell says he encouraged everyone to follow Wahlberg’s lead in how far to take their talk, even locals like Kate O’Brien (Conan’s younger sister) in her film-acting debut as one of the hilariously foul-mouthed sisters.