The Sheldon Museum in Middlebury, Vermont contains a collection of material from the Stewart family. Included is a notebook written by Cynthia containing notes to herself from 1829-1845.
Cynthia seems to have been a girl of rare accomplishments for her day. Her letters, which show glimpses of an unusually affectionate family life, reveal her as being the mainstay to three generations of her family (Private letters of The Stewart family of Middlebury, Vermont. Selected and edited by John E. Stewart, 1968. Sheldon Museum, Middlebury, Vt.)
From her letters to her parents and brothers as well as theirs to her and each other, we get glimpses of an affectionate family life, and many references occur enabling us to place the relatives of the father and mother. It is a matter of regret that so few of these Stewart letters have been preserved. Cynthia had a school friend, Fannie Hine, who was a lifelong correspondent, and it is a curious feeling one has in reading this complete correspondence of a lifetime. Bright, facetious letters for the greater part were these epistles of Fannie Hine, whose home seems to have been at Fishkill Village, N.Y.

Cynthia never married. Her fiancee, a gentleman named Swift, was drowned before a date was set for their marriage (Severance, B. Frank. Genealogy and biography of the descendants of Walter Stewart of Scotland and of John Stewart who came to America in 1718 and settled in Londonderry, N.H. Greenfield, Mass. : T. Morey & Son, 1905, pg. 79-80)