all i have to say is if there is so much people want incest in this game why not make a mod for it with Mr Jets permission
Because it's near impossible.
There are no scenes with the sister so someone would have to create those scenes on their own which would mean recreating the sisters model exactly.
Then they would need to change the entire story to fit that relationship in.
It would basically be making an entirely new game.
The sister has no scenes whatsoever. This isn't a game with the incest removed there is no incest whatsoever so it's not just a case of turning on a switch, it would need a lot .. a LOT .. of work.