When Barack Obama first met his Auntie Zeituni at an airport in Kenya in 1988, his late father's sister told him, "Welcome home," and kissed him on both cheeks. Obama was on a pilgrimage to the land where his African father lived apart from Obama's American mother. Zeituni Onyango then told Obama's older half-sister Auma, "Make sure he doesn't get lost again." What does that mean? Obama asked. As he wrote in his compelling memoir, "Dreams from my Father," Onyango answered, "Don't get lost" is a common expression: "Sometimes it has a more serious meaning. Let's say a son or...