My ancestor Martha Allen born 1795 (married James Murray) is listed as a sister of Thomas Allen, your ancestor. They lived in Houston County GA, then were in Marion and Schley. She died in Schley c1851. This information came from other researchers.
Do you list a father for Thomas? Some researchers list Martha's parents as John Allen Jr. (born 1748 Ireland, came to GA c1770) and Unknown, and him a son of John Allen Sr. who died c1803 in Burke/Jefferson Co GA (widow Jane).