The guy wrote saying it was the fifth time he, his wife, his sister and her husband had a sex party in front of a webcam. That night he knew he and his sister was ready to finally do it but his wife and specially her husband didn't let it happen. The moment was when he starts to finger his sister, went she's ready her husband gets in the way and the opportunity is lost. He doesn't know if it's jealousy or if it's because they are brother and sister.