Up to their knees, they’re identical: laced-up leather boots, pale socks, paler legs, shadow-strewn knees. Then difference begins. On the left, ruffles become a dress. On the right, checked shorts become a white shirt, become a soft bow tie knotted under a boyish face. Soft bangs falling over his forehead, he’s morose. Bows in her hair, she smiles. The little boy and girl holding hands might be twins. But the similitude does not end there. Hung near this difference-parsing, class-identifying photograph by August Sander—an elegant gelatin silver print didactically titled Middle-class Children (1925)—are Diane Arbus’s equally indelible and strangely edifying black-and-white photographic investigations of singles, couples, twins, and triplets made some four decades later. See her famous Teenage Couple on Hudson Street, NYC with their oddly mature faces and diminutive child bodies, or Triplets in Their Bedroom, N.J. (both 1963). The identical sisters are clothed as such: starched white shirts buttoned to the neck like fundamentalists, white headbands bisecting their dark, curly hair. The straight lines of their mouths mimic the line of their eyebrows, running parallel to the sad line of their eyes. Their faces conjure a perversion of Madeline’s famous rhyme: “In three straight lines they broke their …