Let me try to be as clear as possible about Sarah (Souther) Gilliam. She was born about 1790 and probably the first daughter of Michael Souther.
Michael Souther was born between 1760 and 1764, almost certainly in Virginia. The Southers came to North Carolina in the early 1770s. Michael Souther had lived in Wilkes, Burke, and Rutherford Counties in North Carolina. Between 1820 and 1830, he finally settled in what was (in the 1830s and 1840s) Buncombe County, NC. The area is now Henderson County, near the town of Fletcher.
Michael Souther died sometime between 17 July 1834 (when he signed a deed) and 16 Apr 1839 (when a married daughter sold her share in Michael Souther's real and personal estate.)
When Mrs. Sarah "Sallie" (Souther) Gilliam participated in the 1840 Buncombe deed, she was selling her shares in the Michael Souther real estate to her brother Joshua Souther.By the same deed, Joseph Souther and Stephen Souther, her brothers, also sold their shares in the real estate, as did Michael Souther's aged widow Mrs. Agatha Souther. The buyor in the estate sale was Joshua Souther, another son and heir of Michael Souther.
Just to add to the complexity, this same Joshua Souther (the one who was a younger brother to Sarah [Souther] Gilliam) had married about 1820 to the former Dicy Gilliam. Dicy was born in May, sometime between 1800 and 1804. Mrs. Dicy [Gilliam] Souther died 24 May 1849, so she does not appear with her husband on the 1850 census in Buncombe County. The fact of her marriage, month of birth, and the date of death come from the family Bible of Joshua Souther.
Furthermore, the eldest brother to Joshua Souther and Sarah (Souther) Gilliam was named Stephen Souther. Stephen Souther married in Burke County, NC in 1813 to Nancy Gilliam.
This means that three children of Michael Souther married spouses with the Gilliam surname. So if you aren't confused by now, then you aren't trying.