Interesting twist. Certainly, it's an interesting theory.
However, I personally doubt that Nicole was unfaithful to John before MC came into her life, and I don't think Chloe was the product of a previous relationship either and that John simply adopted her. Someone like John, so controlling of everything and so obsessed with his ancestry, I don't think he'd just adopt a child that wasn't his biologically.
And I don't think Nicole has made a move similar to that of Mercedes in the latest film version of the Count of Monte Cristo (2002), where Mercedes (Nicole) would justify marrying Mondego (John) so soon to make him believe her lie that the child she was expecting was his premature child (Chloe), not the child of her former fiancé Edmond Dantes (who was wrongly imprisoned by the machinations of Mondego, because he wanted her all to himself) (an unknown previous boyfriend).
If that were the case, I think the game would have already shown some flashback of Nicole, where she remembered that hypothetical previous boyfriend, but the few past memories of her show how happy her family was when Chloe was little, including a normal parent-child relationship between John and Chloe. On the other hand, a past like that doesn't justify the tremendous impact that her past as Nicole Starr had on her.
However, it should not be forgotten that during the night of Day 7, a drunk Nicole admitted to MC that she had a past as a party girl and that she loves alcohol, which could give credibility to that theory.
On the other hand, just before MC meets John in person for the first time at dinner on Day 1, Chloe mentions that her father has been part of the Magic Grand Real Estate since they were established, around 20 years ago. Considering that Chloe must be 18, plus 9 months pregnant, it seems that John has been working at the real estate agency since at least when Nicole got pregnant with Chloe.
For all these reasons, I highly doubt that there is a genetic relationship between MC and Chloe (to the disappointment of the many incest fans in this forum).
Personally, I think that the blue folder contains information that MC's father is going to be transferred to another city, or that it is he or his wife who does have a real health problem, and not his sister-in-law Martha (who we have not seen yet).
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However, there is a question that we do not ask ourselves:
the tremendous resemblance between Chloe Heavens and Nanci, Abi's dead sister (after all, it's the real reason Beeman has a crush on Chloe). Such a resemblance may support the possibility that Chloe could be adopted or that Nanci and Abi are related to Nicole or John in some way (although Abi claims not to have met John before, and when she first met Nicole, she doesn't seem to have known her before either).
Perhaps Nanci got pregnant by Beeman's stepbrother (who was probably murdered by Beeman himself because he wanted Nanci for himself, but after Nanci's death, he had to settle for her sister Abi) and she concealed a pregnancy until she gave the child up for adoption, and then killed herself out of depression caused both by the public disclosure of her boyfriend's death and by giving up their child for adoption (perhaps motivated by the belief that he could have fled as soon as she told him that she was pregnant, because if I'm not mistaken, his body was found several months later). Meanwhile, Nicole and John would try to have children, but they had problems to achieve it and they finally decided to opt for adoption (again, though, I don't think that's the case because John seems to be obsessive enough about his ancestry and I think the idea of having non-biological children wouldn't sit well with him).
