Wall of text incoming.
I can work with either the split personality theory or the two real twins theory. There is enough "evidence" to, at least, support both. And then some more to expand them:
If they are twins, then their story could be either real, or partially or completely made up. Maybe they were separated at birth, and maybe they found each other some years laters. But maybe they didn't go through all those "twin magic" adventures. Or maybe they did. We can't know for sure.
If they are not twins, then there are other possibilities. It could be that Hannah (or Eve) is suffering from a personality disorder. Hannah only once refers to Eve by name, but Eve talks about Hannah a lot. Maybe Eve is the "dominant" personality, and Hannah isn't aware of anything (or not everything) that happens when Eve surfaces? Or maybe they are fully aware of "each other" and they can communicate via the diary one of them mentions. For example, in a recent Hulk storyline "Doc Green" (aka an intelligent Hulk) is able to "chat" with Bruce Banner using a blackboard.
Moreover, it could be that there's no split personality, but rather a very clever woman who knows how to confuse the hell out of the police and get away with murder. Everything she says could be made up and Eve may be a red herring created to throw suspicion away from her. If so, she must have gone to Glasgow after killing Simon to have an alibi that would only work if there was another person present when he was killed. Basically, she would have set up her alibi by creating a new reality for the scene of the crime (and all her backstory as well!).
As someone pointed out, the alledged life of the twins was basically a modern fairy tale: mistaken identities, people living in the attic (or castle tower, as it may be), a "prince" (Hannah) switching places with a "beggar" (Eve), gruesome deaths (Simon's), etc. Maybe all those stories that Hannah read as a kid made her create this magic reality where she found her nice/evil dopplegangger, Eve. Recall how much they hated each other sometimes, like when one tried to drown the other. Or maybe she purposefully based her made-up cover story on those fairy tales to confuse the police...
Then again, Eve does fail the lie detector test when she claims that she is Hannah. She either isn't who she claims to be, or she is a great liar... OR she does suffer from split personality disorder and actually believes herself to be Eve at that moment. Remember that a lie detector only checks whether the person's reaction to an answer matches what they think is true. If you are completely and absolutely sure that the Sun orbits the Earth, then you'll pass the test, regardless of the truth value of your claim.
What does Eve mean when she says that her sister has gone away? Is Hannah dead? Has she fled? If this is Eve talking, she is pregnant, but she confessed to at least being accessory in Simon's murder. But she was pregnant! That would be completely against the idea of putting the baby (Sarah) first. If it was the other way around--Hannah confessing to the murder or, at least, stalling the police enough for Eve to escape--it would make much more sense.
Either way, both theories have their inherent strong and weak points. we can agree that Eve and/or Hannah is an unreliable narrator. Everything that can't be checked by external sources (for example, whether Hannah or Eve was in a hospital in Glasgow) is up for debate. From their birth to the death of Simon, nothing is set in stone if it comes from the mouth of Eve/Hannah. There are just a few things that we can be absolutely sure about: Simon is dead, and he was killed, and either Hannah or Eve had a child, Sarah, since we are controlling her throughout the game (and "SB" talks about "her mother" and what she did).
Well, actually, now that I think about it, claiming that Sarah, the face we see reflected on the monitor, is Eve's or Hannah's daughter may be put into question as well. She may be just a young girl who has gone through a similar situation and "SB" gave her access to those files to help her understand better why her mother acted like Hannah/Sarah did. But, IMO, this would be stretching it far too thin...
What I just can't work with is the idea of something outside from plausible reality being at work here. I mean, sure: one of the women may come from a parallel dimension, she may be a robot from the future or an alien who took Hannah's image. But there's nothing in the narrative of the game that supports those theories. There's nothing that points at anything supernatural or, at least, anything that couldn't work in the real world as the catalyst for this situation. It's the textbook definition of a deus ex machina, and I honestly don't think it fits here.
Has someone given more thought to this: how can we explain that Eve fails the lie detector test and confesses to helping Hannah getting rid of the body, if she was pregnant and acknowledging her part in the murder would mean that she would be put away for, at least, some time?
Edit: I see some people posted while I was writing this. Guys, remember: whether we find Eve's and Hannah's life feasible or not doesn't matter. The interviewee (whoever she might be) is not reliable. She may be making everything up, but there could still be a pair of twins. Even if it all was true, it's just improbable... but not impossible. Nothing in Hannah's and Eve's narrative outright contradicts the rules of reality. It would be highly, nearly impossibly, odd for someone to pull out those twin tricks, but, hey... is it really impossible to think that they may have?
