The Edward Darley Boit family was from New England, but John Singer Sargent painted the four daughters in the foyer of a grand Paris apartment, deliberately situating them in a locus of uncertainty — perpetually on a threshold, like their roving expatriate parents. Sargent makes you feel simultaneously drawn into and excluded from the sisters’ world, a phenomenon that Erica E. ­Hirshler, a senior curator at the Boston Museum of Fine Arts, explores in her new book, “Sargent’s Daughters.” “From this singular picture,” she explains, “a novel unfolds.”