Just tonight I was reading the holdings of the Missouri Historical Society at St. Louis and came across a reference to what may be what you are indicating.
It says in short that the "St. Louis Star" newspaper of 29 January 1922 contains an article entitled "Former Judge Withrow Recalls Execution of Spy Said to Have Been Engaged to Lee's Daughter. Col. Lawrence Orton Williams and Orderly Caught Inside Union Army Lines in Tennessee." The article quotes Judge James E. Withrow, a veteran of the 78th Illinois Infantry telling how Colonel Williams and Lieutenant Walter G. Peters entered his regiment's camp at Franklin, TN in June 1863 and were later hanged as spies.
Could this be what the other person was intending, and not a brother of Lee? I throw it in just in case it may have merit on this case. From the brief mention I could not determine if Colonel William or LT Peters was the one engaged to Lee's daughter.