To go with my guessing game, here is information on Nellie Grant as well as a brief biography from PBS (this page includes information with the other Grant children as well):
The media adored Nellie Grant, Ulysses. S. Grant's only daughter. She was born on the Fourth of July 1855 at White Haven, her mother's family home near St. Louis, Missouri. Nellie spent her teenage years in the White House, where the newspapers made her out to be a kind of American princess. True, she did seem to live a princess' lifestyle. At age 16, she was sent to Europe, where she studied in London and even met Queen Victoria. On her return trip home, she met a dashing young Englishman, and immediately fell in love.
Nellie married Algernon Sartoris, the son of famed opera singer Fanny Kemble, on May 21, 1874, in the White House. But after the couple moved back to England, Nellie discovered her husband was less dashing than he seemed. Some said Sartoris had problems with alcohol. Others said that he cheated on Nellie, or just plain ignored her. Nellie had four children by Sartoris. The oldest, a son, died in 1876. Nellie was never happy in her marriage. Eventually, she and Sartoris divorced.
Nellie saw little of her parents during the time she lived in England. But when Ulysses S. Grant was dying, Nellie returned to the United States. After her father's death, Nellie and her three children settled in with her mother in Washington, D.C. In 1912, Nellie remarried to a man named Frank Hatch Jones. She died in 1922.