Some examples of pairings I am thinking of:
Mother/Son - they are on a road trip and a mixup at a hotel forces them to share a bed. The closeness and sexy nightware lead to erections and arousal and very little sleep is had. Both agree to keep this a secret, though can't wait for their next encounter.
Brother/Sister - A techy brother has secretly been watching his sister put on webcam shows for some time now. She masturbates and poses nude. At the start of her next show, he walks in and blackmails her with her secret. Her fans notice the new man in the room and his boner, and convince her to give him a blowjob...starting them on a path of debauchery.
Fater/Daughter - a fight with her boyfriend leaves her bound naked to her bed and blindfolded. Her father passes her room on his way to get some water and cant pass up on the oppportunity with his attractive daughter. She only realizes its not her boyfrined when a big hard cock penetraes her and it feels so much better, so he initial hesistation soon dissolves.