The inspiration for this prompt wasn’t anything all that earth shattering. It was the concept of the monster under the bed and seems to have evolved into something about changelings.
To be clear, I am not saying that the brother is a changeling, but it’s possible. The older sibling is currently of an unknown age, maybe still a child, but I see the sibling as an older sister who is now an adult. The conversation feels like an interview on a talk show.
Considering she says everyone “says” she’s crazy this is not meant to be a problem that was solved. Or she’s wrong. She is talking about her baby brother being replaced with someone else. Why would she say that? Did she see something that she can’t consciously remember, but that makes her know something happened to her brother when he was a baby? Or maybe she heard something. Could it be something less obvious and more like her brother went from crying all the time to being extremely quiet?
There is also the possibility that he looked completely different, but all the adults were in on it, so no one acknowledged the change hoping she would eventually forget.
As to the brother, I think the biggest question is, where is he now. This applies to both her actual brother and, if he was replaced, the replacement brother. The brother she grew up with, whether replacement or not, may have distanced himself from her when he was old enough simply because he knew she thought he was fake. Most likely even if he was a replacement, he is unaware of this fact. It would hurt to have your sister repeatedly say you aren’t her real brother.
Now, I did see this glorious prompt once where a mother’s child was switched with another child and she did everything she could to find her missing child. When she did she convinced the creature who took her child to let her leave with her child, but when the creature tried to take back the replacement child she declared that both children were hers and it was absolutely wonderful. So it is possible to go down the route that although she doesn’t acknowledge him as her original brother, she might still have acknowledged him as her brother over time and did her best to make him feel like she loved him despite still not thinking that he was the brother born into her family, basically treating him as if he had been adopted by her as her brother and she doesn’t want to lose him, but she does want her other brother back, too. If that is the case, she might well be on good terms with him regardless of his actual origins.
As for the original brother, if he was in fact replaced, where he is might depend on why he was replaced and who was responsible. As he was a baby, maybe there was a situation where it was a mistake on the part of the doctor. I don’t see that actually happening in modern day, but maybe in a historical story it might have. They could have switched the babies back and no one wants to talk about it, because it hurts, although if she’s an adult now, it probably would have been a good idea to take her aside and explain at some point.
There is also the possibility that her brother died and her parents could not deal with it, so they got a new baby on the black market somehow. This seems unlikely unless they knew he was going to die and thus planned for it or they took her somewhere on a trip and when she returned they had replaced their dead son.
I also considered the possibility of her father having a mistress who gave birth to a child at the same approximate time and switched the babies so that her son would be raised better and that she black mailed the father into going along with it or something. It’s kind of silly, but who knows.
All of these are not impossible in reality, but as stated above, not all possibilities are rooted in reality. A changeling could have been placed there instead of her original brother or the monster under the bed could have somehow done something to him, maybe even replacing him with the monster in disguise. There seems a higher likelihood that he is alive in both of these situations, though he still might not be. Perhaps the story could be about rescuing him . Or perhaps the story is about accepting that she is wrong. I can see the latter being a children’s book while the former could easily be a horror book or a mystery. What kind of story would you tell with this prompt? Is she right or is she wrong?