I had thought about doing this for 18 months before the circumstances happened. As my wife left the house for 3 hours of shopping she said 'My sister may drop something off in an hour or so. Just tell her I went shopping.'
My plan sprang into action. I unlocked the front door, disrobed, and prepared to jump in the shower. I waited in the Master bedroom watching out the window patiently. Now my wife is hot, but my sister-in-law is hotter: bigger boobs (D's) prettier face, nicer ass, and I've fantasized about her often.
The bathroom is down the hall to the left of the main entrance. Leaving the light on and the door slightly ajar would surely tempt my sis-in-law to peek in. The shower is full glass and around the corner, but with two mirrored walls and is fully visible from the cracked door through the reflections in the mirrors.
After 30 minutes my sister-in-law's car came down the street. I quickly hopped in the shower. My heart was beating soooo fast. Would she spy on me? Would she watch me shower? I soaped up and faintly heard the doorbell ring. I started masturbating. I was very hard, trying not to come early, as I was watching the crack of the door in the reflection for what seemed like 5 minutes. Suddenly, out of the corner of my eye, I saw movement. I peeked up through the shampoo and sure enough, my sis-in-law's beautiful face was peering at me through the cracked door. My heart was racing knowing I was being watched. I quickly came with 3 ropes of cum, my best orgasm ever. I gave her quite a show. She must have watched for a minute or so...
I turned off the shower and started drying off, sensing she left (satisfied I hope). I watched as she backed out of the driveway. Amazing. I wonder if she ever told my wife.