Even though they are your husbands parents what they are doing is harassment and invasion of privacy, and they have absolutely no right to do it. And it sounds to me as though it's not because they are so clueless to their own actions but that they are fully aware, at least the father is. If you are able to report them I'd probably do so, you shouldn't feel unsafe in your own house, especially when it's relatives causing it.
I had a neighbour that watched me, definitely not to that extent, but I could see her staring at me through the window, she would take my newspaper from my driveway if I didn't pick it up up within a few hours- I had been wondering why my newspaper never came each week until I saw her take it one day, she's sweep my driveway, she said 'this is my part of the yard and you can't go here', if she heard any sound from the radio or TV at night she'd be at my door, even at 2am, telling me she'd report me next time. Once she came to complain about my TV an when I denied it (the TV was on, but it was actually me saying something loudly at the television that she heard) she went to my window and was shining her torch through to see if I had it on. I had to be so quiet every night and was paranoid walking down my driveway. And it was all because of my age, she didn't want me there because I was so young.

While I don't think it is necessarily illegal (I'm not sure though), it is still wrong. It's intimidation and they shouldn't be allowed to get away with it. Just because they are related doesn't mean they have authority to harass you, bang on your windows and stare at you.