Harriet Rebecca Lane ( May 9 , 1830 â `` July 3 , 1903 ) , was the niece of James Buchanan , the 15th president of the United States , who never married . Harriet Rebecca Lane Johnston ( May 9 , 1830 -- July 3 , 1903 ) , niece of lifelong bachelor United States President James Buchanan , acted as First Lady of the United States from 1857 to 1861 .