Family behavior is learned and repeated- passed on generation to generation. We embed it on many levels as a given, like the air we breathe, and so very dificult to analyze and correct.

She also probably just saw this as sex play, which was automatically wrong, and didn't really want to go into it in her own thinking to see what you all were doing. She ery likely just minimized in her own mind your possession of her slip- figuring other enforcements would suppress any ideas you might have of bringing it into the open.

Threatening extinction was my parents' favorite quick and easy general enforcement. Hugely damaging to me, but it was their own life as well, just being repeated.