All you guys talking about the continuation of human race through incest angle is forgetting that the dad is a biology teacher. He has a particular area of specialty that will come up later but suffice to say, he is more than likely to be very well aware of the futility of the situation. In short, there is insufficient degree of generic diversity within those 4 individual to continue the human species. This is actually an area of some academic research in context of number of people needed to seed an interstellar colonization. The studies have shown you need about 10,000 to 40,000 unrelated people to ensure generic diversity required for a healthy population. 4 RELATED people isn't going to cut it. The dad would know this almost certainly. Saying that there's insufficient degree of genetic diversity would be like saying a drop of water in a 10 gallon drum would be insufficient to fill it up.
So if you insist on continuing to talk about this, that would be to satisfy your own deviant prurient interest, and not because it's a plot matter. Enjoy the fluff and camping/survival advice to come up instead.