I'm a big fan of incest stories, but this little sub-genre doesn't do anything for me. If I'm reading a story and it gets to the point where mother/son, father/daughter, brother/sister sit down and start talking about masturbation, it's an instant turn-off. Especially when it turns into a sex education lesson. The son/daughter/sister feels guilty and are told that it's natural, and everyone does it, and blah blah blah. I know mutual masturbation probably happens a lot more in real families than actual full sex, and it is cool (in real life) but it just doesn't do it for me when I'm reading erotic fiction.
But that's just me.