The Sinclair family papers consist of 62 letters written by members of the Sinclair family and their friends in the mid-19th century.
The earliest items are letters from Thomas S. Sinclair in Steubenville, Ohio, to Ellen Legget of Carrollton, Ohio. Sinclair, who may have been a suitor, wrote of his feelings toward Ellen and his lack of experience corresponding with women (March 6, 1837). In another early letter, John Sinclair related news of a new telegraph line opening between Chicago and Detroit to his brother, Robert Sinclair (March 1, 1848).
After 1850, the majority of items are incoming letters to Thomas Sinclair from his mother, Eliza Cameron, and female acquaintances. Cameron's letters pertain to news from Carrollton, Ohio, particularly related to illnesses and deaths. In her letter of March 22, 1851, she shared an account of the death of her other son, John, and she later asked Thomas how he could move away after her brother's "Death in a foren land" (January 26, 1852). After Sinclair moved to Erie, his mother continued to report on her other children, her social life, and her finances. In her letter of November 9, 1853, she referred to rental properties that she wished to sell (November 9, 1853). She frequently offered her son advice or admonished him for various faults.
Thomas Sinclair also received letters from several female acquaintances during the 1850s. Sarah Sutherland ("Sak") often confronted issues of gender relations and marriage. Following the Erie Gauge War, she expressed her optimism about the progress of women's rights (February 5, 1854). Though she married and encouraged Sinclair to follow suit, she wrote that of her "[intent] to have you for my second husband" (December 21, 1854). Other female friends referred to Sinclair's flirtations and expressed their occasional frustration with his actions; for example, a woman named Nellie called him "Mr. 'Heart-smasher'" (August 18, 1857). Eliza Burton wrote more about her own feelings and aspirations, lamenting the recent path of her life in a letter on March 4, 1856. Burton later worked at a school in Knox, New York, and expressed her despair after hearing of the death of a mutual friend, "Ammi" (July 3, 1858).