Wow, sorry this happened Soundman, but like you said, good thing they didn't cause any irreversible damage to your system. I have become very protective of my humble little setup, and as you also stated - this is the place where most of the folks here can completely sympathize with your concerns over this event. People like your in-laws, and most of the average "Plug-and-Play" consumers out there simply don't realize the time, money and effort that many of us have put into our systems, and that, for some - it's almost a work of art, one's personal masterpiece, if you will, in Home Electronics. I have a simple setup, in comparison, but I still limit those who operate it to only myself and the wife, who I have a.) Trained, trained and re-trained, and b.) gotten a nice Logitech Harmony remote, so that it actually does all the work for her and she doesn't have to figure out how to turn anything on or off.
Their dangerous monkeying around with your system just clearly shows that they do not get what it is that we all here sometimes take for granted.