During night wife got aroused and started to fondle me. Then we did foreplay with her sis next to us but sleeping with bed sheet over her head pretending for sure. Since the bed was shaking lot, we got down and did sex on the floor. Not sure if sis-in-law saw it fully. The arousing fact was, early in morning sis-in-law went to restroom, after her return I went in few minutes later, and saw her panty hanging in the restroom hook hanger and it was awesome wet at crotch area..