Robert Isaac McCartney's Will (Marion County OH, Clerk of Courts, Common Pleas Court, Chancery Record, Vol. 2, 1836-1840, pg. 339) dated December 4, 1839, lists his heirs in the following order, which also happens to be the birth order of his children:

Note, a Catherine and an Elizabeth are listed in Robert's Will along with Robert's eight previously known children. Catherine and Elizabeth may have been children of Robert and Lydia as well. However, no other record or mention of them can be found in any other document or census record other than Robert's Will. One conceivable explanation for this lack of mention of a Catherine and Elizabeth in other documents is the possibility that they died before Robert, but were still listed anyway as his children in his Will, or they died shortly thereafter having never married. If they were not children of Robert, they almost certainly were related to him in some fashion or another.