The Ginny/Jane Davidson who married John McCartney was a daughter of William and Martha (Baker) Davidson. Another daughter named Charlotte Davidson married John Pitzer, Junior, and yet another daughter named Sarah "Sally" Davidson married a Robert Davidson who was from another/almost certainly unrelated (per a lot of research) Davidson family that lived in "next-door" Rockbridge Co., VA. There may have been one other living daughter when William Davidson wrote his will in Botetourt in 1812 (the wording is somewhat hard to interpret as to whether there were three or four living daughters at that time, but only Ginny was mentioned by name).
Note: The Thomas Davidson/Davison and his apparent son Thomas M. Davidson/Davison who were on the 1810, 1820 and/or 1830 Botetourt censuses were from yet another/unrelated Davidson/Davison family in Botetourt.
William and Martha (Baker) Davidson had sons named Baker Davidson (died around 1805; he married Elizabeth "Betsy" Womack), Reverend Andrew Baker Davidson (he married Susan Dorman), William Davidson, Junior (he married a Jinny Davidson who was a sister of the above-mentioned Robert Davidson from the other/unrelated family that lived over in Rockbridge Co., VA) and Joseph Davidson (died by 1814, and he apparently never married; Joseph was the only son who was mentioned by name in William Davidson's 1812 will).
Per some Botetourt tax lists in the 1780s, and per William Davidson's 1812 will, it appears that there was one other (and probably oldest) living son, and he may have been living in a different county by 1812. I have a lot of information on this Davidson family, if anyone is interested (but nothing on the McCartney family).
I was not aware of the referenced 1815 deed. I am not sure who the listed Andrew Davidson was, if he was truly a different man than A. B. (Andrew Baker) Davidson (but maybe he was the "missing oldest son" that I mentioned above). Ditto Jane Davidson, if she was truly a different woman than Jenny/Ginny (Davidson) McCartney. Does the deed refer to that Andrew Davidson and/or that Jane Davidson as being children of the deceased William Davidson? By the way, Mrs. Martha (Baker) Davidson died in 1829 in Botetourt, per a newspaper obit. Also, I suspect (albeit no "concrete proof") that the Captain William Davidson who also died in Botetourt in 1829 (a few months after Martha had died) was the above William Davidson, Junior who had married the apparently unrelated Jinny Davidson from Rockbridge Co., VA. Thanks!