M.F.K. Fisher, The Gastronomical Me
(1943; rpt. San Francisco: North Point Press, 1989), p. 126:
We ate and drank and heard our own suddenly friendly voices over the dark waters, and forgot that Mrs. Feinemann was in her cabin because the captain wouldn't put the Italian wrestler in irons for "making a pass at her," and that Thoreau's grand-niece was very pale from the hemorrhage that had engulfed her earlier in the day.
The anemic lady aboard ship may have claimed or even believed herself to be Thoreau's grand-niece, but she could not have been Thoreau's grand-niece in fact. Thoreau and his siblings (Helen, John, and Sophia) never married and had no children. According to Walter Harding, The Days of Henry Thoreau: A Biography
(1965; rev. ed. New York: Dover Publications, Inc., 1982), p. 21, Thoreau's "Aunt Maria, at her death in Bangor, Maine, in 1881, was the last remaining descendant of the John Thoreau who had immigrated to Boston in 1773."