Recursive

Recursive **** meaning that it repeats across generations. Brother and sister have **** kids together, these kids then grow up and have their own **** kids together, and so on. Or the second round is father/daughter or son/mother, you get the idea. The point being that the **** and babies keep on happening and keeps on resulting in another horny generation, 18 years apart each time (gotta keep the ages legal).

I acknowledge that this isn't exactly an easy kind of story to write, as you're effectively adding characters each time, plus you have to keep the incestuous family tree straight (or keep it tangled, as the case may be). Some of the stories in the **** section have attempted this, but it seems most people lose their steam before the second generation gets anywhere.

This story could go the romance route, as well, though the lust route is also a clear option.