I came home late from work one night and heard my wife in the shower. I immediately walked into the bathroom to let her know I was home and was pleasantly surprised. We have a glass shower enclosure and not only do I clearly see my wife but also her sister. They were showering together and my wife was lathering up her sister. It was so hot to see I got an instant hardon. I had never seen her sister naked before and watching my wife run her hands over her body made it so hot to watch. They both saw me standing there but there was no effort made to stop, cover up or tell me to get out so I took in as much as I could before I thought I might be pushing it.