Barack Obama is mourning the loss of his step-grandmother, Sarah Ogwel Onyango Obama.The former U.S. president took to Instagram on Monday, revealing that the matriarch of his Kenyan family has died. She was at least 99 years old.«My family and I are mourning the loss of our beloved grandmother, Sarah Ogwel Onyango Obama, affectionately known to many as 'Mama Sarah' but known to us as 'Dani' or Granny,» Obama's message began.