I'm the King of the Castle (1989)

Thomas is the 12-year-old son of a millionaire who lives in a big mansion surrounded by woods in France. When his mother dies, his father hires a widowed maid to take care of everything while he is away. The woman brings her only son, Charles, to live with them and, hopefully, be company to lonesome Thomas. The rich boy and the poor one become enemies from the very first encounter. Moved essentially by jealousy and fear, Thomas decides to turn Charles' life into a hell on Earth, especially after their parents eventually fall in love. As the title suggests, he wants to make it clear to the invader who is the lord of the castle.