Goodnight Mommy is a disturbing movie. It's an Austrian film, in the German language. SPOILER ALERT After undergoing cosmetic surgery, a mother comes home to her identical twin nine-year-old sons, Elias and Lukas. Her head is covered in bandages, with only her mouth and eyes visible. She is acting strangely, ignoring her son Lukas entirely, even to the point of refusing to feed him, and tries to makes her other son promise never to speak to his brother again. Lukas too, when he has something he wants to say to his mother, whispers it to his brother, and he asks it for him, so there's no communication between Lukas and his mother. Mom keeps the blinds closed in the house, and does not allow her children to play indoors, or even to speak to one another. The boys become convinced that the woman is not their mother, as this is not the way that their mother would treat them. Their suspicions are confirmed when they come across an old photo of their mother with another woman who is wearing identical clothing and looks very much like their mother. They escape and walk into town, trying to get the priest to help them. Instead, he drives them home and turns them over to their mother. One night, the boys tie the woman who is posing as their mother to the bed while she is asleep and they refuse to let her go until she tells them where their real mother is. She insists she is their mother. At one point, they even burn her face with a magnifying glass in order to get her to talk. She still insists she's their mother. When a couple of adults come over, the cover her mouth with duct tape so she can't scream. She manages to get the duct tape off, but too late. The adults have already left. They superglue her mouth shut. Realizing that she cannot eat that way, they later cut her mouth free. Since she has been tied to the bed for a long time, she wets herself. They free her temporarily in order to change the bedding, but she escapes. However, they set a trap at the front door, which trips her. She falls and knocks herself unconscious. When she comes to, she finds that they have superglued her to the floor, after having dragged her back inside. They threaten to burn the house unless she tells them where their mother is, with Elias (as always) doing the speaking. She tells Elias that his brother is dead, but that it was not his fault. There is no explanation as to just what had happened to Lukas, but apparently, Elias refused to admit that his brother was dead, and mom has been trying to get him to accept that his brother was dead so that they could move on with their lives. Elias challenger her to prove she is their mother by telling them what Lukas is doing. Since mom cannot see Elias' hallucination of Lukas, she can't answer the question. He burns the house down, with his mother and himself still in it, although that's not entirely clear. My assumption is that all three are dead in the final scene. The final scene shows Elias and Lukas walking through the cornfield, where they meet their mother on the other side. Although the movie doesn't make it clear what the accident was that led to the death of Lukas, the movie began with the two boys playing in a lake. Lukas goes under the water, and Elias gets concerned when he doesn't come up right away. But then the scene shows them both walking up to the house, so that scene doesn't really become important, other than in retrospect. During the movie, mom never speaks directly to Lukas but, there too, except in retrospect, the oddness is attributed to the mother who, for some reason, refuses to speak to one of her twin sons.
I read a book that was a very similar plot, called “The Other”, written by Thomas Tryon, also about two boys , one of whom is dead, but still treated like he is alive by the other twin. Like your story, it seems like both are alive for the whole book. The story itself is different from what you are describing in the movie, but the premise is the same, and it is one of those books that you read, and then read through again when you get to the end and know what is really happening throughout the book.