Saw my sister-in-law naked

Few years ago, wife and I were having a new shower and tub put in. So the bathroom was all torn apart for a few days. One of my wife's sisters and her husband live a few miles from us. They said we could come over and use their shower if we need to until ours is fixed. That was nice of them so sure. I would go over every morning to their place and shower before work. Every morning I went the only people there was myself and my sister in law. Her husband goes to work earlier at 6:00 and was already gone when I got there, and she works part time and not every day, so she was always there. My wife also goes to work earlier so she was already showered and gone. One particular day, I showered as normal and was about to leave. Walked by her bedroom and saw her completely naked in her room. She was getting dressed. I was quiet and honestly don't think she had any idea I saw her. The door was mostly closed but I happened to be at the perfect spot when walked by. Saw her from the side for a second and then she turned around and saw her completely from the back. Didn't see the front. But I did look for a few seconds. Naked as the day she was born. I was almost in shock like is this really happening. After looking longer than I should have, I walked away and yelled at her as I was leaving out the front door thanks and see you tomorrow. I have never told my wife. Occasionally wonder if I should tell her or leave it alone. I do feel some guilt over it. Probably won't ever mention it.