I'm the King of the Castle (1989)

Je suis le seigneur du château

Thomas, the son of a millionaire lives a fairly isolated existence, in a mansion in rural France. His father hires a widowed woman to take care of things while he is away. The maid's son, Charles moves in as well, and the two parents hope that the two can become friends but they become enemies immediately after meeting each other. Once their parents fall in love, Thomas decides to make Charles, who he views as an "invader", as miserable as possible.