Complex sentence: A letter from a spurned mistress to her paramour is discovered in the dressing room at one of the estates--a letter actually written to the Princess' uncle, the Vidame de Chartres, who has also become entangled in a relationship with the Queen.
Split into simpler sentences: A letter from a spurned mistress to her paramour is discovered in the dressing room at one of the estates. The letter is actually to the Princess' uncle, the Vidame de Chartres, who has also become entangled in a relationship with the Queen.