Half-sister.
"The family is not expected to move into the White House, though they are very close to the Obamas.
“Maya made many trips to Chicago and elsewhere throughout the campaign, and she and her brother have a great relationship,” said Alan S. King, a lawyer in Chicago who is a friend of Mr. Obama."
http://www.nytimes.com/2009/07/31/us/politics/31sister.html?_r=1