> I've seen references to a Morgan Abernathy in the Lexington/Purcel area. Do you know if this is another brother?
No, I do not believe he could possibly be. I have been given to understand from my cousin that "Aunt Lizzie's" notes are both extensive and exact. They may now be thoroughly cross-referenced by the lifelong efforts of several members of her family, including her great-nephew, Teacher & Historian as well as a Genealogist.
John Morgan Abernathy died, unmarried, on 11 Feb 1892 at the age of 29 years, and there is nothing what so ever to indicate that he ever left Tennessee.
On the other hand, much of the central portion of the United States during that time frame (especially Kansas, Arkansas, Missouri, Oklahoma and Texas) was a cross-roads for representatives from many different branches of the various Abernathy families, including several apparently unrelated ones that only came to this country after the 1860's.
In 1880 there is a Morgan Abernathy in Arkansas who derives from one of the Illinois families, most of whom are NOT at all related to the colonial Abernathy lineages of NC, TN and what is known as the "Southside" of VA, south and east of the Peterboro and the James River. The bulk of the upper midwest Abernathys derive from an Irish Abernathy line out of PA and Fredericks County VA.I discount also the young Morgan Abernathy found 1900 in Spartanburg County, SC: his grandfather was born in NC, and his parents in SC.
Even before 1850 it is a grave fallacy to presume that two individuals with the Abernathy surname residing in the same general area happen to be very closely related until it has been thoroughly demonstrated: there are just too many diverse Abernathy branches spread out all over everywhere. Before 1800, it might be a safer bet but not certain even at that early date. After the opening of the Louisiana Purchase, no. One is safer to assume that they are NOT related until it's proven.