One other interesting point is that initially they did not know they were related.
That set me thinking about how many relationships out there are like this, unknown to the couple.
If you think that this is very rare, then just consider this: a research project in the UK had to be abandoned when they realised that 2 in 5 of the children they were investigating could not have had the father that was stated on their birth certificates.
The Gods are just, and of our pleasant vices
Make instruments to plague us