Nightcap (2000)

"Nightcap" is a 2000 Thriller movie directed by Charlotte Armstrong and starring by Isabelle Huppert, Jacques Dutronc. Mika, heiress to a Swiss chocolate company, is married to celebrated pianist André and stepmother to his son, Guillaume, whose mother died in a car wreck on his tenth birthday. Their lives are interrupted by the unexpected arrival of Jeanne, a young woman who has learned she was almost switched with Guillaume at birth.