Brother and sister are inherently good kids, but no matter what their kids do in the house, their parents seem to not notice or care, as long as they aren't breaking a particular set of rules. This leads them to experiment with each other and eventually have the sister's "normal" seating position in her brother's lap as the subtlety of what they are doing in front of their parents turns into exhibitionism.
Sounds like a family lifestyle choice to me. There have been a few arrests in isolated areas without a whole lot of media coverage.
In all likelyhood it is more common than not.
It's not a bad story idea, but rather than making the incest blatant, and the parents complete morons, the story (from a story perspective) would be stronger if the kids start trying to see what they can get away with, but then decide they enjoy having sex in front of unaware observers instead.
In that case, the kids could have set sitting on the couch watching TV with the family (with the traditional blanket over their laps), while the parents are clueless. Add in a subplot where the mother thinks something is happening, but isn't sure, so she keeps looking hoping for confirmation, but ends up getting caught up in it and getting excited herself (i.e. a slow introduction to a semi-family share scenario).
The kids could then expand their game to having sex in restaurants, in stores and school. Getting caught, and having to talk their way out of it, would only ramp up the excitement level.
After playing this game, someone (hopefully a teen girl or female teacher rather than an adult male) demands they get to join in, in exchange for her silence.
For each person they then subsequently add to their 'game', that person/couple then begins playing similar games themselves. It'll be tricky portraying this without going over the top (i.e. making completely unbelievable), but with care you could make it work by keeping the sex low-key throughout, but you'll have to show how teens delaying sexual satisfaction actually ups the kink factor, think 'tantric sex'.
Around that point, you could have the mother move from 'watching' her kids have sex without her preoccupied husband noticing to joining them, and then slowly joining in with sex with their friends.
That leaves the father-daughter kink out of the story, but it makes it more believable and eliminates the jealousy/trading-lover to others issues. Adding in their getting caught and having to dance around being reported will keep the story tension high, as will their keeping everything a secret from their father (possibly make him a (non-born again, merely impotent or asexual) minister adds an extra twist.
I think the story could work, but I wouldn't make it a complete hump-fest (though if you do go that route, then forget all my cautions, as the whole point in porn is getting off as soon as possible, rather than developing the story).
It sounds promising, unfortunately it's not my kind of incest story. I prefer pseudo-incest stories where the whole challenge is getting to having sex in a one-on-one true love scenario.
