Invisible Child (1999)

Invisible Child is a 1999 film starring Rita Wilson as a mother who imagines she has three children when she only has two. Fearing his wife may be institutionalized because of her mental illness, her husband goes along with this charade as though it is perfectly normal. The 10-year old daughter assists her father in facilitating the delusion. The youngest child seems to really believe that he has a sister named "Maggie". They hire a nanny who initially goes along with the family's unusual situation, but she becomes concerned about the effects on the real children and reports the family to child protective services. A legal battle ensues, ending in victory for the family. It is also revealed that the real children were not harmed by pretending about "Maggie". In the middle of the movie the husband explains that he tried to take his wife to the best psychologists in town. They told him that she was mentally ill and would have to go to an institution. He didn't want that to happen, so he went along with it for five years. His wife hears him carrying on a conversation with "Maggie" one night. The next morning she states that "Maggie" has died. Apparently, hearing the aforementioned conversation helps the wife out of her mental illness and allows her to finally let go of her imaginary child.