Hannah and/or Eve's love of fairy tales, since fairy tales have several similar tropes to their lives, especially Eve's and her Wicked Stepmother, Prince and Pauper situation, and locked up like Rapunzel situation.
There is a very simple reason why we never find out for sure whether Hannah/Eve are actually twins, split personalities or one woman trying very hard to evade conviction: the game is set 21 years after the interviews were recorded, and given the protagonist is Eve's daughter, she'd obviously already know which of these 3 is correct. Sarah's concern is why she/they did it.
In order to get pregnant, Eve sleeps around and in the process is infected with an STD. She later sleeps with Simon and is impregnated by him, which means that they weren't using protection. This means that it is entirely possible that Simon and later Sarah would've unknowingly been infected. It becomes worse if you believe that Eve is Hannah's split personality, since it means that Hannah has unknowingly given herself an STD, then unknowingly passed it on to the husband she loved and her daughter Sarah. All in addition to possibly triggering her miscarriage and not even knowing it.