Her genealogical outline is here. Mark Leepson, in Lafayette - Lessons in Leadership from the Idealist General states the Marquis de Lafayette had ongoing relationships with the Comtesse d'Hunolstein and Diane Adélaïde. The Comtesse d'Hunolstein broke off her relationship, left public life, and retired to a convent while the Marquis continued his relationship with this Countess.
From batguano.
Hair went straight up in the 1760s and 70s; now it is spreading out. Countess Diane also appears to be wearing a sheath dress with a tent bodice.
Keywords: 1783, Vigée-Lebrun, Comtesse de Simiane, Damas d'Antigny family, Simiane family, Countess, French, mistress, Marquis de Lafayette, bouffant coiffure, ribbon headdress, square neckline, bows, tent bodice, quarter length close sleeves, ruffles, under-dress, modesty piece, flared under-sleeves, waist band