No doubt, you have the McKelvy information by now.
Mildred Angeline "Milly" Harper was the daughter of William Uel and Anna (Fargason) Harper.
William Uel Harper's parents are undocumented at this time.
Anna Fargason was the daughter of Johnson and Mildred (Hogg) Fargason/Ferguson.(Ref: First Families of Henry Co., GA, p. 251.)
Mildred Hogg was the daughter of Major Lewis Hogg and "Clara".(Ref: Newberry Co., SC, Will Book L, pp. 339-341.)However, in First Families of Henry Co., GA, page 250, her mother's name is given as Clarissa (Smith) Hogg.
I do not know the origin of the Waters name in the family.There is a Waters family from SC living in Cass County in 1850 -- Sebastian and Pheba.Sebastian was a miller.Perhaps, they are related in some way.