As noted in Gene Dorris' Dorris-Net newsletter Volume 5, Issue 4, page 3 & 4, Robert Thompson married Mary Dorris who is presumed to be the sister of William Dorris Sr.
The son of Robert Thompson may have been Moses Thompson - Moses was listed as an Executor on the 1818 Will of William Dorris Jr. The other two Executors were the sons of William so naming Moses is a strong indication of a close familial relationship.
There are few documents about Moses Thompson that I found in an internet search.
History of Edgefield County, a book written in 1897 by John A. Chapman
See page 10 - This Moses may not be the same man given the date of 1756 for the reference but possibly a brother of Robert or some other relation?
These particular Thompsons and the Dorrises remained close while in South Carolina and at least one Thompson family can be found living close to the Dorrises in Georgia (Jackson County and then Cherokee County).
On the 1830 census for Jackson County Georgia a Mary Thompson is listed with 2 females in the household: 1 age 40-49 and 1 age 70-79. The adjoining neighbors of James Cochran, Jacob Braselton, John Randolph and Prosser Horton Jr. (the son of my 5th great grandfather Prosser Horton), place Mary Thompson's residence in the present day Braselton-Hoschton general area. James Dorris is listed about 2 pages back - we now know that James lived in downtown Jefferson. (James is on page 313, Mary is on page 316.) There are other Thompsons living in the same area: Sherrod Thompson (page 313, age 70-80) and a Richard Thompson (page 314, age 50-60). The adjacent neighbor of Richard Thompson is Archibald Jackson and Archibald is the man that sold James Dorris land to be held in trust for Mary Thompson.
Then, on the 1840 census for Cherokee County Georgia, Mary Thompson is listed living near James Dorris in District 892. James is listed on page 21 and Mary is on page 22. Mary lists 2 females, 1 age 40-50 and 1 age 50-60.
One might presume that Mary Thompson is the widow of one of the children of Robert and Mary Dorris Thompson. It might even be possible that the older female listed in the 1830 census (age 70-79) is Mary Dorris Thompson. With William Dorris' birth date estimated to have been around 1835, if this Mary is his sister, she would have been at least 15 years younger than him which is possible. Most likely this Mary Thompson is a daughter or daughter-in-law of Robert and Mary Dorris Thompson.
(This page was updated on 10/25/2020)