Peter and Rebecca Harris: mid-forties denizens of Manhattan’s SoHo, nearing the apogee of committed careers in the arts—he a dealer, she an editor. With a spacious loft, a college-age daughter in Boston, and lively friends, they are admirable, enviable contemporary urbanites with every reason, it seems, to be happy. Then Rebecca’s much younger look-alike brother, Ethan (known in the family as M...