I am also a decendent of Willis Kirkley, although from looking at the 1860 and 1870 census, I'm not sure whether I'm related to Willis Sr. or Willis Jr. It appears that Nancy Mozingo with with the elder Willis in both 1860 and 1870 census.
My great grandfather is John P. Kirkley, son of Nancy Mozingo and Willis Kirkley. John P. appears in the 1870 census in Hale Co., AL (see Willis Kirksey). In 1880 census he is in Fayette Co., AL with mother Nancy, awidow.
Your Susan is a brother of the younger Willis, and daughter of older Willis.
Have you found anymore information about Willis Sr. (1803), Willis Jr. (1820) or George Kirkley b. between 1807-1810? I'm pretty sure that Willis the elder and George Kirkley b. 1807 are brothers. Possibly Thomas b. 1803 NC is another brother too.