Ann Patchett’s new novel is an American family saga involving six children, 50 years and too many coincidences to count. The premise is straight out of John Updike — a writer she admires — but her eye is on free love’s fallout, not its thrills. As the title hints, she’s interested in the larger family units that itchy-footed spouse-swappers inadvertently create when they do the dirty on their kin.