In the OP:What is even the point lmao?
Incest fans want only one of these.
"At the beginning you get to select the relationship you have with Ayumi, being either sister or step-sister."
The point? Well, some are fans of the incest kink, some not. And there is the issue that some sites used by game developers allow 'step-cest' but not incest content.
Bottom line for this game is that the developer doesn't force either relationship. Each player can choose their personal preference of the two relationships.