1850 census of Bath Co., Ky, division 2, enumerated 21 Aug, lists on p. 86 as family #581:
Sarah McIlvain, 38; William, 25, farmer; Gilbert, 20, farmer; Caroline, 17; Rosy A., 16; Archibald, 13; Isabella, 4; Louisa, 2; John, 6/12. All born in Kentucky.

Either Sarah is really older than 38, or William is not her son. And if she is really the mother of Virginia “Jennie” McIlvain born ca 1857, then her age is unlikely to be much older, in fact, later censuses suggest that she may really have been only about 35 in 1850. So the most likely relationship is that William is her stepson, the son of an earlier marrage for James.

1860 census of Bath Co., Ky, Owingsville precinct, enumerated 13 July, lists on p. 115 as family #833:
Sarah McIlvain, 44, farmer; Isabelle, 16; Louisa, 11; John, 10; Virginia, 3. All born in Kentucky.

1870 census of Bath Co., Ky, Mudlick precinct, Pealed Oak P.O., enumerated 17 Aug, lists on p. 33 as family #232/234:
McIlvain, Sarah, 55, keeping house; Louisa, 21; John, 20, farm hand; Virginia, 12. Rayburn, Barton, 69, farm hand. All born in Kentucky.

1880 census of Bath Co., Ky, Tanyard precinct, enumerated 5 June, lists on p. 8 as family #66:
McElvain, John, 30, farmer; Millison, 31, wife; Thomas, 4, son; William, 2, son; Sarah, 63, mother; Jennie, 22, sister. All born Ky Ky Ky.