At first, I thought she was the sister because they were at MC's mother's house, Natalie. But during the sex scene, she calls him Dad, so I change to daughter. To be honest I have no idea who she is.
As a matter of fact, I have no idea who is who in this game. The game is a mess regarding relationships. I think the dev doesn't care about it and neither to the story. It seems that the relationship change is only there to please the incest fans, cause even with the "daughters" the dialogue doesn't work properly.
I only play it cause I like the visuals and the sex scenes, cause the story is just an excuse for a fuckfest.