Angels and Insects
(1995)

The "Stunning Revelation" is that Eugenia Alabaster Adamson (Patsy Kensit) has a long-running incestuous relationship with her brother Edgar (Douglas Henshall). When he catches them in bed, her husband William Adamson (Mark Rylance) leaves the aristocratic villa with his loyal assistant Matty (Kristin Scott Thomas), for a new expedition to South America.

Thanks Tilman!