I agree it doesn't make sense. The only thing I can think of is that if the relationship continued, you could have a second generation of siblings who wanted to mate and that really could be pushing things too far. This way, there would never be a full sibling for the offspring.
One of the well-known examples of incest over several generations is of course the Egyptian royal family. You would think the Egyptians were smart enough to work out that it really wasn't a good idea, but it seems stopping anyone else getting a foot in the royal door was more important than being fit and healthy. Here's a quick snapshot:
Quote:
Inbreeding within the royal families of ancient Egypt often led to stillbirths, along with defects and genetic disorders. The results of inbreeding certainly may have taken a toll on Tutankhamun himself.
The same DNA tests that identified King Tutankhamun's family also showed that the Boy King had a number of illnesses and disorders, including a deformed foot caused by a degenerative bone disease that forced him to walk with a cane. He also had a cleft palate and a curved spine, and was probably weakened by inflammation and problems with his immune system. King Tut's problems that were related to inbreeding most likely contributed to the boy's death, but were not the immediate cause. On top of everything, he had a badly broken right leg and a bad case of malaria. This, combined with his other underlying health problems, is likely what killed King Tut. The pharaoh did not successfully produce a successor and he was the last of his dynasty.
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(From a site called medicalbag.com.)