I love the idea of incest, but it feels even better when there are no gender barriers. The ideal incest family consists of members who are all bisexual. Here's one idea.
A young guy (18-20) finds himself alone in the house and raids his parents' porn stash for an afternoon of masturbation. One of the tapes features guys wearing women's underwear. He finds this really exciting and goes upstairs to his parents room. Oblivious of time he starts trying on his mother's bras and panties. His mother, having returned home, discovers him resulting in shock and embarrassment (and loss, of course, of son's hard-on). Mother assures son that his 'fetish' is perfectly OK and natural and starts to help him with choosing underwear and make up. All the time she's talking to him in a very suggestive way, telling him how excited men and women would be to see him. Despite his embarrassment her talk results in a raging hard-on. She leads him by the hand to his own bedroom where his father is lying stroking a serious erection. Mother changes character - he needs punishing for interfering with her stuff. His penance - to suck his father. Punishment over, his father reciprocates to the obvious delight of his wtaching mother. Eventually the scene progresses to a son sandwich in which the cross-dressing son, his cock buried inside his mothers ass, is buggered by his father.
How about some other ideas/comments?