Okay so here is my theory (super detailed):
Two girls, twins, were born and one appeared dead. The mother, having only wanted one to begin with (and the other with the possibility to die or appearing dead at least) was taken away by the midwife, Florence. Florence realizes she’s made an error (or the baby comes through as she is “taking care of” the dead baby) and she decides to raise it as her own, and names it Eve. When Eve is 9 years old she finds her “mother’s” diary that tells her more about her life (she’s a war-widow from her twenties, never remarried and the story of Eve) and on the day Florence dies, “falling” down the stairs (still don’t know if this is truth or not) Eve goes across the street to meet her sister, the healthy twin, Hannah.
Hannah hides Eve in her private playspace, the attic. There they decide to live as the same person, coming up with all kinds of secret ways to communicate and a big list of rules. They use knock codes, hidden messages in their cat’s bell, and keep detailed diaries so they can live the one life.
Fastforward a few years and in comes this boy, Simon. Hannah falls head over heels for him, breaks the rules and gets pregnant from him. Eve in her panic to match her sister tries everything she can to get pregnant, but fails. This causes the two girls to slowly grow apart. The parents find out about the baby and a whirl-wind wedding is arranged. 8 months later Hannah loses the baby. Soon after that her parents die from “food poisoning”, Death Cap mushrooms, which is suspicious considering her father was a Mushroom expert, but Eve seems to be sincerely upset by it in her interview, or it was Hannah and I couldn’t tell :P One way or another, parents die, Eve is in the attic at the time and it’s a summer heat wave, eventually they find the bodies, and Hannah and Simon move into her parents house.
Around this time Eve decides she needs to gtfo and become her own person. She leaves the attic, gets a tattoo and starts sleeping around and becoming a lounge singer at a bar in Oxford.
Simon randomly ends up in Oxford for a conference and meets Eve there, singing, and is like “Holy sheet, you look just like my wife, ‘cept you’re blonde (wig) and I totally dig Blondes more, wanna bang?” and Eve is like “noooo… noooo… we shouldn’t… oh okay fine let’s bang.” so they do and Simon and his miracle sperm knocks up the other sister (Eve) who thought she was infertile from all her failed attempts before.
So, Eve goes to Hannah and is like “Shit, so, i banged this dude at a bar I work at.. totally my boyfriend and not at all your husband.. and i dunno wtf to do about this cause suddenly single mom with no insurance.” Her sister, Hannah, is like “Well fuck, grats i guess, sucks cause I can’t seem to get pregnant anymore, but whatever, guess i’ll just tell Simon and you can move in here and we’ll help take care of you and shit.”
So Hannah goes to Simon and says, “Hey guess what? I have a long lost sister you never knew about, yeah she’s my twin, and I want her to move in with us, cool? Cool.”
Simon understandably freaks the fuck out.
Hannah’s birthday rolls around (coincidentally also Eve’s cuz twins) and Eve is all like “HAPPY BIRTHDAY! oh btw I’m pregnant.” And around now Hannah is starting to get a little suspicious. Simon then gives her this bitchin’ mirror and is all like “it’s unique and one of a kind and shit and happy birthday.” and she’s like “Oh. Uh. Great. Thanks……. btw Eve is preggers.” and Simon once again freaks the fuck out. Eve can’t take everyone freaking the fuck out and is like “Crap on this, I’m going for a drive.” and she heads up to Glasgow. Simon also leaves to the bar, and Hannah just sits there with her stupid mirror, wondering if her hubby banged her twin who wears the wig.
A few hours later Simon walks in and for whatever reason Hannah felt like trying on Eve’s wig, “she enjoyed pretending to be me” and Simon thinks (understandably) that she’s Eve. He confesses with the baby and everything that he loves her and he wants to be with her and that he will leave Hannah. He then pulls out a duplicate of the “unique mirror” he made and gave to Hannah earlier. Hannah sees this, and realizing he made two, one for Eve, flips her shit royally, and smashes the mirror. Simon is all like “DOUBLE YOU TEA EFF, MATE?!” and tries to grab “Eve”. Hannah slices at him with a shard of the broken mirror, and slits his throat open in her panic/distress. Sometimes before or after that he punches the shit out of her eyeball. He dies. Hannah’s like “oh shit.”
Meanwhile, in Glasgow, Eve can’t drive for shit, and runs into some dude with no insurance. He tells her “WTF YOU DON’T HAVE INSURANCE?!” and she’s like “I’M PREGNANT” and he’s like “OH SHIT, HOSPITAL! STAT!” and she’s admitted. Eve calls Hannah (post Hospital and nap in the car) and is like “Oh hey sister, what’s up? I almost killed my baby cause I can’t drive.” and Hannah is all like “Oh hey sister, nothing much, murdered Simon, glad the baby is alright. Wtf should I do?” Eve is like, “Oh hey, you know what, it’s cool cause I have this whole witness thing in Glasgow and since no one knows I exist you can just say it was you here and we’ll have your alibi!” and Hannah says “Yeah! Awesome idea! Come help me with the body!” SO Eve drives back down and helps Hannah cut up the body and put it in bags from their kitchen and they shove them in the basement. They come up with the alibi and Hannah goes to the police with her story (black eye and all). The police ask her questions over the next few days, and at some point (for some reason) Eve replaces her in the interviewing process (which is why no Black eye)
Eventually they go to the police saying they found the body, and Eve cleverly left the watch (broken from the fight) and it shows the time of the murder, giving Hannah the alibi of being in Glasgow (when it was really Eve). The police eventually decide she’s the number one suspect (I presume) and put her up on a lie detector.
They ask her for her name and she says “Hannah” and immediately police know something is up and busted. The rest of the videos are Eve explaining their background, their story, how/why Hannah killed Simon, and about the plot.
Tada! (We’ll know if I’m right soon :D)