1: Sperming My Nieces by Dr Scribble I like lots of Dr Scribble's stroke stories. They're enough tongue-in-cheek to give a smile to the basic implausibility (as of most good stroke stories), but not too much. Here there's a prohibition that makes it exciting, since he can't slide his cock back and forth repeatedly in a niece's pussy. There's fun at the "edges" this forms to separate it from ordinary sex.
2: A Very Lucky Man by Vidi Veni Not an unusual fantasy, maybe, to have ongoing sex with your nubile stepdaughter -- with her and her mother's enthusiastic consent. This story is about as believable as this kind of scenario can get, and the sex descriptions are detailed, evocative and flow well, without getting crude and frantic.