I wouldn't. I think that the Nancy in the 1870 census and the 1880 census may be the same. What I'm worried about is that there doesn't seem to be continuity in the records. I would be more comfortable if there was a clear marriage document or the 1900 census. Andrew Sherman, born in Pennsylvania, his parents born and died in Pennsylvania. He's dead before 1910 but there are no records for him. How does he get to Graham County? How does this family end up in South Carolina? Where are his records? As an outsider, I just don't see the connection, and I worry that the name Nancy Martin is common.