(This is mostly for myself. I might replay it to pay more attention to things -especially to the police siren- and maybe find more missing pieces.)
As far as my theory (based on the videos I saw) goes: (It's been almost three weeks since I played, so my memories are a little fuzzy on some things.)
You are Sarah, Hannas daugther. She is pregmant at the time of the interview/s.
Hanna is the woman with the ponytail, Eve is the woman with open hair. They are identical twins. (Both probably have psychological problems.)
Florence (midwife for the twins mother) lived across the street of the partens and her husband died in the war, when she was around twenty years old. She never remarried, but always wanted kids. When the twins were born, she told the mother her firstborn (I think I remember it was the firstborn) was dead and took the (not dead) baby home. Eve was never allowed to leave the house and was home schooled. She watched Hannah, when she played outside, wondering about that "reflection". (That whole part sounds crazy. Did no-one ever notice a girl at a window in a house, that wasn't supposed to have a kid in it? I do believe that no-one wondered about a midwife carrying babystuff in and out the house though.) Florence had written a diary and Eve found and read it, but burned it later. (So not really proof for the twins theory instead of split personality.)
One day (when the twins were 8? 10? I don't know/remember) Florence fell down the stairs (might have had help, even though Eve said it was an accident) and Eve went outside and met Hannah. Eve began living in the attic of her real parents home. Eve and Hannah switched places when they wanted to (for eating time for example; "ate for two but they didn't mind because it didn't show on me" or someting like that; or school-(the one who was better in a field/class would take the test-; but even for dating -he must have thought we had terrible bladder problems with how ofter we went to the bathroom to switch places-). They wrote a detailed diary, so they wouldn't say the wrong things or not know important things and so on. (One day Eve was down for the bathroom, which wasn't so bad, because Hannah was in her room and their mother saw her in her pajamas/underwhere and asked why she wasn't dressed, because they were about to go somewhere as a family. Eve ran into Hannahs room und Hannah came out "a moment later" fully dressed up (appropriately for the occasion). They also had rules.
Hannah broke the rules. She wanted Simon to herself (no more switching places). And she got pregnant. Hannah and Simon "had" to marry, because of the baby. Eve had lots of sex with strangers (because she couldn't with people they knew) to get pregnant, but it didn't happen. "For the first time it wasn't like seeing a reflection of myself" (or something like that).
Something I'm not sure on: Around that time (Hannahs pregnancy) their parents died from mushroom poisoning, even though their Dad was an expert and it couldn't have been an accident. Eve claimed she slept through the whole thing because she was ill. (I'm guessing one of the twins poisoned the parents. I didn't find a video that prooves it though.)
Since the twins were "separate people" for the first time, Eve left and had to get her own life. She became a singer (in London/Portsmouth? Don't remember if that is correct.).
Hannah lost the baby late in the pregnancy. She didn't get pregnant again after, presumably due to complications from the miscarriage. The marriage went on for 15 years and was occasionally rocky.
Simon went on business trips and ended up in the bar Eve sang. They talked. Eve found it hilarious, that he said she was completeley different from his wife.
For their wedding anniversary Simon (had) made a "unique" mirror and gifted it to Hannah (and gifted/had gifted another handmade mirror that looked the same to Eve and told her, he wanted to be with her/liked her more -or something like that-). Hannah got angry about the "unique" mirror and broke it. Simon hit her. She took a shard from the mirror and slashed in his direction to scare him of (that is what is said anyway). She happend to hit his throat and he died. It was Hannah's plan to hide his body in the cellar and she dragged him there. Eve had an alibi for them because she was in Glasgow.
Another part I'm not entirely sure on: Hannah got pregnant (surprisingly) somewhere around this time and decided she wanted Eve in her life and she wanted to tell Simon all about her, so they could be sisters. She didn't tell Simon about the pregnancy yet. She was waiting for the right time. When she told him, he wasn't exactly happy about the baby.
The baby is all that matters.
All in all, the split-personality seems more likely, but it's boring. :-)