Originally Posted by Nightshade

Sit em both down and put it out there on the table and embarass the fuck out of them by being blunt about their obvious relationship. Grab a handful of condoms and lay them on the table with both of them there and simply say, "I know you two are fucking so these condoms are for you to use. If my sister gets knocked up I am going to make sure you never use your dick again." Then simply walk away not saying another word about it.