My sister in law ask me to fuck her but my wife is in the next room

My sister in law is single and she is still studing at university, so she has no too much time for fun. Sometimes, when she needs to disconnect, she ask me to visit her in the night while everything's quiet. The deal is that I have to stay, be quiet and don't look us in the eyes. My part of the deal is clear, isn't it? My wife use to be in the next room, so we have to be careful. It's really hard because she moves like a godess. This time she fucks me doggystyle and let me creampie her tight and little used pussy. And she even doesn't mind if I record it.