Don't fuck your family means don't fuck your family.
But my mind recalls an excellent essay by an anon a long time ago explaining why in-laws, step-siblings, etc became so frequent in porn. Something about an easy plot setup and a few other things.
Coco', I tan and lotion, sunsets over the ocean
Look up, hot air balloons, all good things come in twos
Work them out at the gym, dive in and take a swim
Everybody love the twins
Everybody love the twins
So juicy and so ripe, you wouldn't believe
I give 'em different names, Mary-Kate and Ashley