Once you realize what the school is for and that Mashiro is actually two different people, you realize that Fem!Mashiro just killed her brother in the womb.
In a similar vein, it is stated that when a student misses the class three weeks in a row, they lose all the progress they made and give up. This is heavily implied to be miscarriage or stillbirth.
Given the revelation that none of the characters technically exist yet, it's not clear how Kureha could've been raped as a child or Sou could be in an incestuous relationship with his sister if they haven't been born yet. Perhaps this is less that they've experienced those things and more that they're actually products of those things. Doubles as Fridge Brilliance.
