As I see it, there are two issues in incest: The genetic risk and the distinction between parent and lover.

Incest with a dependant child is wrong because you can't be both a parenta nd lover. Incest that produces a child with a blood relative is wrong because of the higher chance of birth defects.

However, when you eliminate these two categories, I don't see that there is anything inherently wrong with incest. The mere fact that two adults happen to already be related is nothing against a sexual relationship as far as I'm concerned.

I would be very surprised if it ever actually happened in my case--I know of no living blood relatives and I can't picture any of my relatives by marriage being interested.