Watching s*s In Law Turnabout

After my recent experience of watching my s*s in law get off in the bathtub, I find that most every time I think about it I get aroused and need to rub one off to satisfy the state of excitement. Because her job requires some travel, she sometimes spends 2-3 day per week with us and has complete access to our home.
The wife is out, and I am laying back in the bed with my hard cock in my hand, slowly stroking. I have a porno on the bedroom TV, but I am really thinking of the next time I get a good chance to watch her. Unfortunately she has been taking showers the last few visits so I have not had a chance to do the hidden video.
I am really working the moment having almost climaxed a couple times when I glance up at the TV on the dresser. In the dresser mirror next to the TV, I see the bedroom door which I swung shut but didn't completely close and see a slight shadow move then stop. Thinking the wife has come home and caught me stroking off again, which ultimately leads at least a BJ, I just continue and wait for her to enter, but the door doesn't open. I look a little closer and can see blond hair, not brown, through the crack in the door, and the movement has stopped. It clearly is not my wife and the only other person it could possibly be other than a burglar, is my s*s in law. I stop for a moment and realize she is watching me!
I once again start to slowly stroke my now extremely rock hard cock, only at this point, there is no way I can avoid a climax knowing she is watching me the same way I watched her just a few days before. After maybe 20 seconds, I explode a huge load shooting cum all over my stomach and chest (almost hitting myself in the chin).
I can barely see her outside the door, but I know she is smiling as she slowly, quietly walks away. I clean up, turn off the TV and go back into the house and meet her as she is bringing in some stuff from her car. As she greets me, she is obviously pretending she has just arrived and is walking in the door, but I know better.
I know this will never become a sexual thing between her and I, and don't want it to. The question now is what's more exciting, me watching her or her watching me? Next time she is doing herself in the bath with her eyes closed, will it be my climax she is thinking about while reaching her own as she rubs her pussy under the water?