There was also another Margaret Rankin, daughter of William and Mary (Lourance) Rankin. She was b. 15 Aug 1816 and d. 2 Apr 1883 in Iredell County. Actually, I'm not positive that they lived in Iredell, it might have been Lincoln County.
But this Margaret was the niece of the Margaret Rankin who m. Thomas Witherspoon. This younger Margaret Rankin m. Lawson H. Abernathy.
If this is your family, please let me know. I have quite a bit to share with you - although the research is far from complete.