My wife and I talk like this all the time. My wife has to start it, though. If I start it, she gets upset. Most conversation revolves around her sister, too. She loves to think that I am cheating on her with her sister. My wife loves when I pretend she is her sister and she tells me all the things to do to her.
Outside of the bedroom, if I even were to accidentally touch her sister, there would be a double murder.
Looking seems to be okay, though.