It seems to me that the John Davis family from 1850 in Smith county is connected based on where the children were born.
Nancy Davis appears to be the same person in Elizabeth Davis household in 1860 census. But I don't think Ann and Nancy where children of Mary A McGee as she would have been abt 15 years old when Ann Davis was born. Mary may have been a Davis and married Mr. McGee, Ann and Nancy could have been her siblings or nieces ? John Davis her brother as age and place of birth about right ? Don't think Ann and Nancy were John's daughters either.