Also, as I read the post, I believe that the OP thought that it was implied that it would be just the two of them. OP didn't mention as they had before that they set the "just you and me" boundary.

I would like to state that he was absolutely in the wrong here and it should have gone without needing to be said that it was supposed to be the two of them. I am assuming that the Dad pushed back on OP reserving the room with their own money because he had always intended on bringing the step-sister along. He probably didn't mention it because he either hoped OP would be okay with it as he was already there with her OR he honestly thought that he was in the right, spending time with both daughters without making one feel left out.

He was, and is, in the wrong. No doubt.