Laura Miller meets Lizzie Post and Daniel Post Senning, two 30-something great-great-grandchildren of Emily Post and the heirs to her etiquette empire, to talk about the challenges of promoting good manners in “a time when jeering, insults, and other forms of impoliteness emanate in a steady stream from the highest office in the land” – and of resisting the desire of (some of) the public to peg them as finger-wagging schoolmarms.