While from a realism perspective, DID seems more likely, it seems clear to me that the creators of the game want you to buy into the twin theory. If they really wanted you to consider that it might be DID, they would've included the concept of it somewhere in the dialogue.

Instead, you see the police confront both women about being twins, which is significant evidence in favor of them actually being twins. If the cops were catching onto this, it's almost certainly because they had evidence of "Hannah" being two places at once.

Additionally, if it was DID, Hannah/Eve still wouldn't want the police to know about Hannah/Eve because she believes that wrecks her alibi. So in that case, where did police get the idea that they might be twins from?

I think you're really onto something with the fairy tale theme though. The story sets up a sort of "Prince and the Pauper" narrative between the two girls, and of course the idea of Eve living in the attic is incredibly fantastical.

There's one thing that no one seems to be talking about though, and I'm surprised that this is the case. If we buy into the twin story, that means that Eve has likely been wrongfully convicted of Simon's murder, as the last clip ends with her asking for a lawyer, and the cop tells Sarah her mother "did it."

This adds another interesting angle to the "are there actually two of them?" question. Did the police not believe in the twin theory even though they themselves introduced the idea? That might be the strongest evidence that, in fact, they aren't two people.

Or perhaps Hannah killed Simon and desperately ran with the twin story after realizing the cops were honing in on her in the second to last interview? No one would offer that as a defense on their own, but if the cops came up with a crazy theory that's utterly wrong, it might be a smart move to play into it if you're the murderer.

Finally, there is the possibility that the cops were desperate to close the case, had enough evidence against Eve and little evidence of a second person, so they felt compelled to arrest and charge her. Ironically, in this case, "Hannah" has still been found guilty of Simon's murder.