There are plenty of incest games, I think this one works better as intended, with MC not actually being a member of the family he's trying to seduce. So many games are sanitized with cheesy "roommates and landlady" edits, but this one is actually written with the intent that "childhood friend" and "friend of my mom" angle are the true story and thus their interactions made sense in that context. There's no reason why Katie would shy away from a sexual relationship with MC, why would she? He's a hot guy and she's not related to him. If it makes sense as written, and it's not written as a family story, then I see no reason to change it.