Savannah Purchase — Jane Aiken Hodge

As children, Juliet and Josephine often played the innocent game of impersonation. They were cousins, but they looked enough alike to be twins.

Life and war separated them, but the years had not dimmed the astonishing resemblance.

Now Fate suddenly threw them together again--two beautiful, desirable women playing out a deadly masquerade.
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UK CORGI Edition:

A CAPRICIOUS ESCAPADE LEADS TO UNFORSEEN ENTANGLEMENTS

Cousins who might be twins, Juliet and Josephine are separated by the Battle of Waterloo, to meet again four years later, on a desolate bluff by the Savannah River. Both are orphans now, but Juliet is penniless, while Josephine has made a marriage of convenience with a rich Savannah planter. Each in her own way is desperate, and when Josephine proposes that Juliet play, once more, their childhood game of impersonation, she is driven to agree. She takes her cousin's place, deceiving even her estranged husband, whiloe Josephine tries a mad gamble to rescue Napoleon from St Helena.

Plunged into the social whirl of Savannah, and on the great plantation of Winchelsea, Juliet finds the stakes higher than she realised, and learns all to soon just how little Josephine has told her.