Okay I absolutely LOVE the camera idea. Even if you only own one, make them think they're everywhere, and you'll have them boning in the bushes, meeting at parks, etc. Maybe you could schedule a meeting with a genetic counselor who could discuss the implications of first cousins having babies, and then take them to an institution to volunteer with people with severe disabilities. You can temporarily avoid all family gatherings, or you can require that the two stay right next to you.
Now in my family, I'd just tell Grandma. That would embarass the hell out of the two horndogs.