It could be, for example, that a new wing was built into the house, and the serial killer just happened to be loitering around the area in which this new wing was built. But I don’t suppose there’s any foreshadowing for that.
Regarding the more plausible option of the enterability of the house changing, the door is an electronic door that only accepts a certain keycard during night but can be freely opened during the day. The narrator was in a daze until morning, and thus the door changed its mode on its own. The narrator breaking the key with his hand suggests the “key” is not an ordinary metal key, as no human has the strength to just casually crush one in their hand. This lends credibility to the idea that they key is actually a keycard, and thus the door is actually an electronic door which can be programmed to switch its requirements of entry in any arbitrary manner.
Foreshadowing to the effect of the houses being different? Early on, the narrative directly mentions rain falling on the windows, yet a prior red states that the existence of windows might as well be denied. Clearly, the murders happened in a separate house that has no windows! …yeah, that’s not going to work. Oh well.
Anyhow, if foreshadowing is what you want, how about this. The narrator does not consider himself a person. This is foreshadowed by the narrator alternating his gaze between the floor and his hand. The first and the last time, he simply neutrally comments “hand”, but the second time he looks at his hand, he disregards it as “another pointless knick-knack”. This means that when the narrator listed the two people in the house, he did not consider himself a part of the two people. Why did he say “myself”, then? Because the killer had somehow assumed his identity in the narrator’s mind. This is not a case of a split personality, the narrator merely considers himself an object, and believes that the person he formerly was is now the serial killer.
Or perhaps, the narrator’s assessment changing from “hand” to “not a person” back to “hand” indicates that the narrator has a condition that makes him sometimes unable to distinguish between objects and people. Thus, at the time he remarked only two people are in the house, he was unable to perceive the third person, the killer, as anything but an object.
While we’re at the topic of changes in whether one counts as a person or not, when it was specified there are only two people in the house, perhaps the killer was in a state of coma or brain death or other state in which their level of consciousness is somewhere between non-existent and vegetable. The killer then wakes up, which is a sight that the narrator simply cannot explain because he thought it impossible, and proceeds with the killing.
People have brought up the idea of the narrator being an accomplice to the crime. My impression of the narrative certainly agrees with that. The narrator talks about something being relevant to the truth of the case before a case even happens, suggesting foreknowledge. At the end, he laughs. He remarks that what I assume is him dying is something to be expected because “he decided to go through with it after all”.
It doesn’t quite stop there, though. He keeps saying these truths aloud for no reason. He refers to red as the language of truth in his own narration. He is amused by the final thought of writing down the fact that he can’t break the key with his current strength, even though that apparently was largely inconsequential, if Vyse’s answer to that particular mystery was indeed correct. So it seems he knows the score regarding the status of the game as a mystery that works with red truth, and he finds it funny to add in one additional puzzle. This leads me to believe that the narrator is not only an accomplice to the killer, but an accomplice to… you, the Sorcerer, the territory lord of this story! Note that the narrative before the story speaks of a device with which the Sorcerer may look at the truths of a story that is not his. The “story” we are to solve is a meta layer inside a meta layer. It is an imaginary scenario crafted by the narrator in his plane of reality, which the Sorcerer shows us through his telescope or whatever. This imaginary scenario is purposefully made to be an unsolvable mystery, as suggested by the highly unusual behaviour of the narrator I described above. As such, because this whole scenario is fictional even in-universe, it need not follow conventional rules of what is possible. The killer is the narrator’s imaginary creation who he conjured into the house with his mind, and thus does not need to enter after the declaration that only two people exist in the house. He can simply start existing there and kill the two siblings.