Hello, distant cousin: I descend from siblings of David Abernathy & Christina Forney and I have your answer.
"Many postings I find do not show her as a child of David Abernathy and Christina Forney, but a couple have."
That is because your Sally was never a child of Christina Forney.
"I am trying to find out who her parents really are. She is my 4th great grandmother, and I descend from their first child David Rudolph Conrad."
Her Father was David Abernathy, son of James Abernathy (drowned 1785 in NC) & Elizabeth Cox. Her Mother was Martha "Patty" Abernathy, daughter of David Abernathy Sr & Ann Turner. Sally's Father died shortly after 18 Jun 1795 and left a will, which unfortunately named only one child: James. Patty was sometimes known as "the widow Bizzell" but not because she had ever been married to a Bizzel. It was because when David's Father was dividing up his estate among his grown sons, David, apparently the eldest son, was granted what had once been called the Bizzel place before James Abernathy purchased it.
While there were three David Abernathys on the 1790 Lincoln County NC census, only knowledge of the names of wives and all three of the couple's children distinguishes between them.
The 3 Davids were 1) David Abernathy Sr (1726-1814), wife Ann Turner, will dated 24 Aug 1808; 2) David Abernathy Jr (1759-1838), wife Christina Forney, daughter of Jacob Forney Sr, buried Giles County TN; 3) David Abernathy (cir 1755 or before, d. 1795), wife Martha Abernathy, daughter of David Abernathy Sr.
The list of children of David Abernathy Sr & Ann Turner includes Joseph Abernathy (d. 1827), sp Ann Williams; Robert Abernathy (d. 1824), sp Sarah Nichols; Martha Abernathy (d. cir 1817, md David Abernathy (d. 1795 NC); David Abernathy Jr (d 1842 TN, sp Christina Forney; John D. Abernathy (d. 1843 NC), sp Susannah Marie Forney; Turner Abernathy (d. 1845 NC), sp Louisa Daisy Abernathy, daughter of Robert Abernathy & Ann Nance; Nancy Abernathy (d. 1847 NC), sp Peter Forney; Moses Abernathy (b. 1767 d. ?), sp Susan Bynum; William Abernathy (d 1850 NC), sp Winifred Kimbrel; Miles Abernathy (d 1863 NC), sp Susannah Parr; Elizabeth Abernathy, Mrs Ephraim Perkins.
The list of children of Christina and David Abernathy Jr includes Dionysia Mrs David Roper (d. 1853 Giles Co., TN); Ephraim Perkins Abernathy (d. 1843 Weakley Co., TN), md Mary Polly Freels & Elizabeth Dickenson;Milton Abernathy (d. 1841 Giles Co., TN) unmarried; "Sina" or Aseneth Mrs Matthew Monroe Johnston, (d. 1861 Giles Co., TN);Littleton Forney Abernathy (d. 1856 Weakley Co., TN), sp Rachel Cuthbert; John Young Abernathy (d. 1849 Giles Co., TN), sp Matilda Sharp Alexander; Robert Gilmore Abernathy (d. 1884 Green Co., MO), sp Martha Fry; Drucilla Young Mrs James Orville Alexander (d. 1830 Maury Co., TN); Susan Alexander Mrs William Riley McGee (d. aft 1870, probably Giles Co., TN).
The list of the children of David & Patty Abernathy of the Bissel place includes Clara Abernathy (?-?); James Abernathy (d. 1850 Perry Co., MO), sp "Franky" Abernathy, daughter of Jones Abernathy & Elizabeth Rutledge; Martha Abernathy (?-?); Sarah "Sally" Abernathy Mrs Peter Rudolph Conrad (d. 1847 Perry Co., MO); Joshua Abernathy (d 1849 Perry Co., MO), sp Martha Abernathy, daughter of Jones Abernathy & Elizabeth Rutledge; Turner Abernathy (d. 1840 Washington Co., IL), sp Margaret Nelson; Grief Abernathy (cir 1793 d aft 1824, location unknown); Nancy Abernathy Mrs John Capps (d. aft 1811, probably Mecklenburg Co., NC); David Abernathy (b c 1796 d bef 1810 NC); 4 un-named sons who died bef 1800 in Lincoln Co., NC.
Thanks go to Loris Jean Delapp Biggs, an elderly descendant of James & Franky Abernathy, for the given names of the apparently eldest son of David & Martha "widow Bizzel" Abernathy, and the necessity of resolving the conflict posed by her supposed descent from Christina (Forney) Abernathy.