Her French-language Wikipedia article is here.
From Wikimedia.
This is the height of Directoire fashion - a loose garment cinched by a band tied a little above her natural waistline with tight sleeves.
Keywords: 1797, Desoria, Constance de Théis, Constance Pipelet, princesse de Salm-Dyck, Salm-Dyck family, Princess, French, curly coiffure, ribbon headdress, scoop neckline, quarter length tight sleeves, waist band, natural high waistline, close skirt