I have James b.1746 and Sidney b.c.1750's children Mary b. 1771, Maryan b.1772 and John b.1777. Cornelius b.1783 - no reason to hide his parentage if he belonged to them.
If his lore, proven right, was that he was adopted by Brandons, we can't rule out that neither of his parents was a Brandon, just part of the Withrow-Brandon overlap of friends and family. If there is a male Withrow death c.1783 (the war was still going on) that might be an indicator.
And of course centuries later and we still all know James Withrow was a philanderer... Where was wife #2 Margaret Etherington in 1783?
I have Withrow correspondence and genealogy I'll get out and the will for somebody or other's spinster sister that bothers me.