On the other hand, if it is to remain a fantasy, you could have a lot of fun with it. My ex-father in law spied on my by reading my partners diaries while we were dating - not just what we did, but what I told my partner about what I did with celebrities I met. Must have been a massive shock to read and take in at the time, but he used it to his advantage at three separate times in my life, and now I am being threatened with his involvement again.
I suppose I am a bit different to your daughter in law, being submissive and easily led.