I can answer a couple of questions off the top of my head. In 1860, Nancy was still in Smyth County with Jane, George and a daughter, Matilda, age 2 (this would be the Sarah M. Barnes) she is living with in the 1880 census of Fayette County).
1860, Rachel is in Wythe County with son, William, age 8.
1870 Nancy is in the home of Joseph and Caroline Creger in Fayette County. George and Martha have their own home, Rachel is with now 18 yr. old William, Jospeh and his wife Rebecca, John B. and his wife, Nancy are all living in Fayette County.
There seems to have been at least two groups of Powers in Virginia. The group in Campbell County and the group in Smyth amd Wythe Counties.
I am fairly certain that Caroline was the daughter of Nancy, but George not sure, nor am I sure that Matilda was a daughter. If she was, she would have been born when Nancy was 40.