When I was younger mom and dad were divorced, so it was just mom, my sister who was two years older and me. One summer the weather was quite hot and my mom and sister decided to be very liberal and just walk around the house nude. Of course I was kind of modest about seeing them that way, with nothing on. They both teased me and said how I should join them. I worked up the nerve and naturally I got an erection. Instead of being shocked or reprimanding or anything by my arousal, they were both delighted with this and it was easy to let one thing lead to another.
Very quickly I was having intercourse with both my mom and my sister. They were both wild and I could tell enjoyed being nasty, and I had trouble in keeping up with them. That summer we had huge amounts of sex and we did everything imaginable. Then we all slowed down as time went on, but we continued to do this together for the next several years. Now mom is remarried, my sister is married and so am I. Still, we all managed to discretely have sex about once year for old time's sakes and it is so wonderful.