Here is your link to the Civil War Letters of two Follett brothers "from Illinois", John M. Follett and William Follett.
My research shows John Follett was likely born in Crown Point, Essex, New York on March 18, 1832 to Abraham Follett and Lorraine Meacham Follett. John married Hortense B. Hill in 1857 in Knox, Illinois. He wrote his wife Hortense some wonderful letters, as did his brother William Follett ("Bill"), who in one letter referred to himself as Hortense's "Aff Bro" which probably means "affectionate brother". I cannot see that William was related to her other than by marriage. But perhaps he felt like a brother to her through her marriage to his brother John.
William wrote his sister-in-law Hortense and his parents frequently, but he did not survive the war. A letter was written by a superior to his father, A.Follett, announcing Williams' death in the battle of Resaca, Georgia in 1864.
John had two daughters with Hortense that are referred to in his letters, Fanny and Flo (Florence). I have no reason to believe that John did not survive the war and return to his family.
Another brother "Mell" is mentioned in the letters. This was a younger brother Melville Follett, born June 7, 1836 in Moriah, Essex, N.Y. Mell was in poor condition in Chattanooga as described in an 1863 letter written by his father who visited him there. I am uncertain about his survival of the war.
These letters truly bring this period of history and the Follett personality to life. They make wonderful reading.