It's because of patreon. In the main menu click on "Enter a code" and enter the code pelican, this will unlock incest. Patreon hates incest that's why all of them are censored.
Then they are lucky they are slipping by because one report is enough to get those taken down, Patreon is really against incest.
Enter the code realstory in the main menu. The wording is so odd because of Patreon, they don't even allow step siblings. The dev can't talk about it here because Patreon is watching.
In the main menu you see a "codes" option. Click on that and enter the code "realstory" , this will unlock the incest path of the game.