This cheesecake portrait demonstrates her relationship to King Friedrich Wilhelm II of Prussia. Her German Wikipedia article is here. It calls her "the Prussian Pompadour."
Countess Wilhelmine wears a long sleeved jacket that appears to be trimmed with lace and a feathered hat over a moderately high coiffure in this 1776 Therbusch portrait. Her dress lacks mechanical supports, such as panniers.
Keywords: 1776, Therbusch, Countess, German, mistress, Friedrich Wilhelm II, feathered hat, long straight coiffure, scarf, jacket, long tight sleeves, cuffs, full skirt, flounces, gloves