Harriet Lane was the daughter of President Buchanan's sister, and after the death of her parents, she and her sister went to live with him. He introduced her to fashionable circles, and she served as his First Lady. She married only after the Civil War, to a Maryland banker. Both her husband and her children died young, and she lived out her life in Washington D.C. as an active and wealth participant in the social scene.
For more information, see the EN Wikipedia article Harriet Lane.