You can trace the Brisbon/ Brisby family through the personal property tax lists of King William County. (The Library of Virginia will send a copy to the nearest public library to you which has a microfilm reader--usually your county library). I was not researching the name when I read the microfilm, but I did note the following:

Ned Brisbon was a "Mulatto" taxable on his own tithe and a horse from 1804 to 1817.
Eliza Brisbon was taxable on a horse in 1834.
Matilda Briston/ Brisby was taxable on a horse from 1845 to 1851.

Note that there is a separate list for people living in the Pamunkey town (since they were not taxable on their persons) and Ned was not listed there. Perhaps the family joined the tribe later.
Paul