Margaret's brother Alembe may be ALLEN B. WILLIAMS.
Website- Sons of Dewitt Colony, TX
I also found another reference to "Allen B." Williams:
DeWitt Colony Land Grants 1825-1832
Tract #172 / Allam B. WILLIAMS / M / 5-16-30 /10/ 5-10-32
I found Allen Williams, on the 1790 Rockingham, NC census #537A.
This could be our elusive Allen B., Allumbee, etc. I also found John Abbott and two Edward Williams. Maybe Edward had a son Edward????
There are also two for James which is the other brother mentioned in the notes for John Abbott and Margaret Williams-Abbott.
I also noticed a Leven listed in 1790 Guilford, NC but he would be too old to be Allumbee's son, perhaps a brother, uncle or cousin?
I also have another question. Do you know if there might be a way to see if any of Alembe's descendants have ever applied to DAR or Sons of the Revolution?
I found this:
North Carolina Revolutionary War Soldiers
You searched for Williams born in North Carolina in 1757
89919. Williams, Allenby
Allumbee Williams was born in Dorchester County, Maryland in 1757, and moved to Guilford county, North Carolina in 1780 where he enlisted in the North Carolina Continental Line.
Excerpt from John Abbott notes:
John Abbott was born in Dorchester County, Maryland on February 16, 1755, and moved to Guilford county, North Carolina in 1780 where he enlisted in the North Carolina Continental Line John Abbott grew up near Chesapeake Bay, MD. 
It's possible the John and Allenby's families lived near each other.
In page 5 of the military is a sworn statement of Sol. Solomon Mitchell that John Abbott and Allenbee Williams left their homes about the year 1781 for the purpose of enlisting in the Rev. Service. They were gone upwards of twelve months, then they returned, and they brought him their discharges which he read and recollects they were honourable discharged, which were signed by Gen. Green. 
I find it interesting that throughout this account John and Allenbe are referred to as "they", which implies they were close friends, possibly cousins.
John Abbott was in the Revolutionary War. His record states "he enlisted on 1 June 1781 at Guilford Court House in the county of Guilford, state of N. Carolina for the term of 1 year, in the company commanded by Captain William Little, company commanded by Major James Armstrong.
John and Allumbee spent the first 2 months of their enlistment "headquartered at the High Hills of Snatee, South Carolina".
John Abbott was discharged on 1 June 1782 "at the big springs near Dorchester and about 18 miles from Charlestown". 
I would assume that Allumbee was also discharged at this time.
About 1803 John Abbott and his wife Margaret Williams
moved to Sumner County, TN where he says he lived for about 25 years,
at which time he moved to Green County, IN where he died in 1836. 
[Note A: John Abbott's February 1833, Greene County, Indiana, Declaration for Pension states that at the time of his court appearance he had lived in
Indiana 'for nearly five years' (1828-1833),
Tennessee 'about twenty-five years' (1803-1828),
and the 'remainder of the time in South Carolina and North Carolina' (1780-1803).
At some point in this time frame Alumbe and some of his children went to Texas, while John, Margaret and their family migrated to Indiana.
It is believed that John Abbott married Margaret Williams, daughter of Edward Williams, in Dorchester County, Maryland, prior to his moving to Guilford County, North Carolina, in 1780, with his father-in-law Edward and brother-in-law Allumbee to establish a new family frontier foothold.
It is also believed that John's wife Margaret did not travel to North Carolina with her husband John, but did so at a later date with her mother, her and John's first child James, and her brother James and sisters Rebecca, Mary, and Elizabeth." 
Does mention mother by name. Alumbe's wife and family probably stayed behind as well.
North Carolina 1787 land records show John and Margaret Williams Abbott living relatively close to land owned by Margaret's father Edward Williams.
On September 15, 1787, John was granted land for his service as a Revolutionary War soldier on Roach Creek, a tributary of the Dan River. This action was recorded on page 60, North Carolina Grant Book, Volume I - April 1786 to September 1806.
I would assume Alembe received a similar land grant.
The North Carolina 1790 Federal Census shows John living in Rockingham County's 'Civil Division 169' and North Carolina 1800 Federal Census shows John residing in Rockingham County's 'Civil Division 445'.