Glass House: The Good Mother (2006) on Netflix

Plot summary

After their parents die, Abby and Ethan Snow are taken in by Eve and Raymond Goode, who recently lost a son named David. The house stands beside a lake. Ethan's room sits near the master bedroom and has its own bathroom, while Abby is sent to the tower, the room farthest from him. The children are given little freedom: any phone call needs a code they have not been told, and Eve has double locks put on every door that leads outside. Raymond says David drowned in the lake. In the basement Abby comes across an old bed, with David's name carved into the post beside it, and David's old bedroom is kept shut to her. She asks Ben Koch, a police officer who has known the family for years, to take her and Ethan away.