You don't say the name of your great grandmother, but I presume she is the M. Jane Hemley per the marriage license for marriage of Moses Mann to M. Jane Hemley from Dec. 1871,
The 1830 Will of John Mann, (b. c. 1793) Abbeville, SC names Moses Mann as one of the executors of his estate. Gilbert Mann is a witness to the Will. According to the Will, John Mann's wife was named Margaret and one of his daughters was also named Margaret. The 1830 Census for Newton County shows Moses "Man" age 47 living with Margaret Man, age 53, so she could be his sister, niece, or sister-in-law.
1811 Will of Jain (Jane) Smith, Abbeville, SC names her four daughters to include Maragret Man and Elizabeth Man. Witnesses to the will are Robert Mann and Gilbert Mann.
1811 Gilbert Mann was also a witness to the Will of Michael Smith, Abbeville, SC.
Moses Mann also served as administrator of the estate of William R. Mann who died in Newton County, GA in 1853.
The 1830 Census for Newton County shows Moses "Man" age 47 living with the older woman, Margaret Man, age 53, who, considering Margaret appears to be a family name, could be his sister, niece, or sister-in-law.
Moses Mann also served as administrator of the estate of William R. Mann (wife's name was Sarah) who died in Newton County, GA in 1853.
The Wills I mentioned are all available from South Carolina Archives, Digital Collection. The URL:
http://www.archivesindex.sc.gov/onlinearchives/search.aspx