Probably a daughter of either James McClain and Sarah Bailey, or John McClain and Pheobe McCarty, but I don’t know which one. Estate or deed records in Bath Co. might resolve the question.

Bath Co., Ky marriage records list the marriage of J. J. Grayson and Susannah McClain on 27 Aug 1842.

1850 census of Bath Co., Ky, division 2, enumerated 16 Aug, lists on p. 74 as family #492:
Goshua Greyson, 27, farmer; Sousan, 35; Elizabeth, 11; Sarah J., 7; Emily, 5; Rebecca, 2; unnamed son, 2/12. All born in Kentucky.

1860 census of Bath Co., Ky, Owingsville P.O., enumerated 6 June, lists on p. 18 as family #131:
Joshua Grayson, 38, farmer; Susan, 41; Elizabeth, 17; Jane, 16; Emily, 13; Rebecca, 10; David, 8; Alace, 3. All born in Kentucky.

Note that their son Sylvanus Butler does not appear in this census. This suggests that the age of 12 recorded for him in 1870 is wrong.

1870 census of Bath Co., Ky, White Sulphur precinct, lists on p. 35 as family #237:
Grayson, Joshua, 41, farmer; Susan, 50; David H., 18; Emely, 2; Laura Ella, 13; Sylvannis, 12; Eldradar (? probably Eldridge, but there is no tail on the next-to-last letter), 46, farmer. All born in Ky.

The ordering suggests that Emily is a granddaughter.

1880 census of Rowan Co., Ky, Cross Roads precinct, enumerated 1 June, lists on p. 18 as family #145/162:
Grayson, Susan, 59; Sylvanus, son, 18. Both born Ky Ky Ky.

Also on this page are the families of their son David Henry Grayson, and the Alvin Ramey family, whose daughter Joanna married Sylvanus within a year.