Forgotten Twins (2004)

Mathieu Bretagne, an abandoned child who has been brought up by foster parents, has become a petty criminal. One day he incidentally discovers that he has a twin brother, Adrien, who is the (adopted) son of a well off family. Revolted at first, Mathieu kidnaps Adrien, holds him prisoner in a boathouse on the port and takes his place in the family. But he soon realizes his attitude will lead him nowhere. Instead he sets Adrien free and both decide to go in search of their biological parents.