Walked In On It

I walked in on a situation that I don’t want to be a part of and wish I didn’t know about. I have kept this secret for a few months and want to tell someone but also don’t want to be around for the trouble it will cause. Over the summer, my wife’s sister got married and her and her husband moved into a new house that’s a couple towns over from us. My wife asked me to drop off a few boxes of things at their house to help them get started since it is their first home. I showed up after lunch time on just a random weekday and her car was there so I walked up to the door and knocked, she didn’t answer but it was open so I started carrying the boxes in. After I was done I looked around for her and she wasn’t in the house so I looked out back thinking she was by the pool. Well, yes she was by the pool. But when I looked out she was completely naked and sitting on the lap of a large black man. It was very clear from her movements that she was fucking him. I didn’t know what to do so I continued to watch and the ended up switching positions and he started taking her from behind. I left before they noticed me and haven’t said anything. But they had only been married about two months at that point and she was already cheating. I don’t want to say anything as I don’t want to get in the middle.