Unless I've been sadly mistaken (and misled) for the last few years...
Breeding sister to brother, whether its people or cattle or little fuzzy gerbils, is not a good idea over many generations.
My point was that incest (when practiced over multiple generations) can lead to some major problems in isolated populations.
The cheeta is a very good example of this in nature. Very narrow gene lines and slowly declining population.
Bees, ants, and house flies are closer to the point.