I believe this is Sarah Hopton, wife of Nathaniel Russell a wealthy 19th century merchant and trader. This portrait hangs on the first turn of a the large free standing staircase in their home of 1808.
Correction; The lady of the house in the painting is Mary Rutledge Smith painted by George Romney. George Romney painted this portrait in England during fifteen sittings between January and May of 1786. The painting was passed down through the family, ultimately finding a home here. Special thanks to Valarie Perry, Historic Charleston Foundation and Jim Boyle the 5th great grandson of Mary Smith.
Charleston, South Carolina.