If I were you, I would move out to a place of my own. He has to prioritize his children. They have a right to the amount of time he is giving them. They also both belong in beds at their father's residence. Neither his daughter, nor his son should have to sleep on a couch. Kids shouldn't be shifted around to accommodate a girlfriend.

This man sleeping with his 9 year old daughter is beyond weird. He should tell her "No, we won't be doing that." However, you don't belong in the middle of the argument that he will then have with his daughter. She thinks you are taking her father away from her, and she has a point. Ideally, you really shouldn't be there on Saturday nights. That is little enough time that this man spends making his children his priority. I wouldn't want a man who would do otherwise.

If you and this guy are serious about each other, then he needs to find a 3 room apartment. Only 5 months into a relationship is a bit soon for that. Maybe you have a friend or relative who could put you up on Saturday nights? Where were you living before you met him?

The welfare of his kids has to be your partner's first priority . . . if he's any kind of a man. He really should have thought this through before inviting you to move in. You and his daughter have become rivals for his attention. That's a bad spot for you to be in. You don't want to be competing with a 9 year old who gets her father for one day a week. That's putting yourself in a very unflattering light.