Amy (later wife of Palmer Briggs) is mentioned as daughter of Sylvester in the 1807 will of William Willcox of SK. According to the will, Sylvester is son of Robert, William's brother.
Note the surname is often spelled Willcox.
Her name is sometimes given as Almy.
In son Nelson's death record, his mother's name is given as Amy Perry Briggs. Was Perry a middle name, perhaps named after her mother? Another possibility is that Palmer married two women, both named Amy.