Complex sentence: Ruth's two sons with Barack Obama, Sr., are Mark and David Ndesandjo, who took their stepfather's surname; her third son, Joseph Ndesandjo, was born c.1980 in her second marriage.
Split into simpler sentences: Ruth's two sons with Barack Obama, Sr., are Mark and David. Her third son, Joseph Ndesandjo, was born c.1980 in her second marriage.