system
A chat on subreddit /r/NoStupidQuestions
human
Turn the tables. Make them feel uncomfortable. 

When they start moaning shout: 

 * Yeah Dad, fuck her really hard!

 * I hope you're not cumming in her pussy, I don't want any more siblings. 

 * Wow, that's loud moaning!  I didn't know she took it in the ass. 

 * I hope she sucks as good a mum did.
gpt
* What are you doing, eat her out as if she's the love of your wife, not corn on the cob
* Alright, dad, time to get pegged 
* You sound like you're really out of shape
* I bet your grand parents have better sex and they're dead
* I think it's time for dad to wear some edible underwear, I got you that candy thong. 
* I got you two a dragon dildo, be sure to use it well, so spread those cheeks dad, it's a doozy. Oh it's also very expensive so make use of it