there are very few of ones where they don't know they're related because 1) generally people here don't understands how tension in stories work, and 2) when the fetish is incest it's kinda necessary to KNOW they're related because otherwise it's just a vanilla story. the "we can't do this, this is wrong" requires knowing.
but yeah, it would be great for the story, more layered and more tension. I wish more incest games tried to include not knowing even a little bit. like with having the classic costume party where identities are missed by at least one side.