The face that should settle the question
A stranger in the house is frightening because you know they do not belong there. According to the announced premise, Other Mommy proposes a nastier problem: Bela forms a relationship with a sinister entity in her home that looks uncannily like her mother. The proposed threat does not simply enter a familiar space. It borrows a familiar face.
That distinction changes the question from who is in the house to whether Bela can trust what she recognises. A child should not have to investigate whether the person who looks like her mother is safe. As an idea, the premise makes that ordinary shortcut to trust feel unreliable.
Why the double matters
Jessica Chastain is set to play both Bela’s mother and Other Mommy, the entity’s manifestation. The announced casting makes the resemblance more than a detail in the description: both roles have the same performer.
That gives the film a sharp conceptual hook, not a guaranteed scare. If both figures are presented through the same face, appearance alone cannot do the work of separating reassurance from danger. The horror would have to live in the gap between recognising someone and knowing who—or what—you are dealing with.
A double can remain disturbing or become a simple guessing game. Whether the film sustains Bela’s uncertainty, and what it ultimately does with her relationship to the entity, remain open questions. That is the test of this premise: can a familiar face stay unsettling after the initial shock of recognition?