Reporting from Nyangoma-Kogelo, Kenya— To secure an audience with the Obama family matriarch at her farmhouse in western Kenya, you are told to pay respects at the local seat of power. This is a run-down government building where the district commissioner, a scowling man in a black suit, receives you without warmth. You've come to see Sarah Onyango, you explain, the woman referred to as "Granny" by the president of the United States. You are coming with the blessing of the president's half brother Malik Obama, you quickly add. District Commissioner Boaz Cherutich, who controls the woman's 24-hour security detail,...