I cannot find anything about her.
Enlarged by half by gogm.
This appears to be a "Dolly Varden" dress described here or a conventional dress that was influenced by the Dolly Varden style. The dress is unusually trim for the era.
Keywords: 1869, Lejeune, Marquise, French, straight coiffure, pony tail, headdress, chemisette, high neckline, bow tie, buttons, jacket, lapels, long close sleeves, cuffs, waist band, natural waistline, over-skirt, bustle, under-skirt, flounces, robe à la circassienne, Dolly Varden dress