Wife’s sister, Jana, was a few years older than us. Once while visiting us she was in the guest bedroom with the door shut (but it had no lock on it). Not thinking I opened it and saw her standing, bare breasted in her panties. She yelped and I shut the door. She had nice pink nipples like my wife.
Years later while at my in-laws, we were in jack & jill bedrooms (bedrooms separated by a shared bathroom). She & her husband in one room, me and wife in the other. Jana was taking a shower, our door was unlocked, I walked in to sneak a peak. Nothing happens but I did fantasize about fucking her.