Last winter I lost all the work I'd done on this particular line, and have spent the last several months trying to reconstruct. My recollection is of finding Lucinda & David Allen on a 1880 census listing them as brother & sister. For the life of me I cannot find them on any 1880, and without that or some cemetery records (I have not found Abercrombie [or variations] listed in any Fannin Co cemetery, so I suspect they may all have been buried on family land).
By thinking David & Lucinda were not bro & sis, I have to go with the theory that Isabella was actually Lucinda Isabella & was listed with both her widowed father, James H, & her grandparents in 1850 MS.
If anything come up to disprove or support my theory on Lucinda and David Allen, I will let you know.