I started thinking about this today. One of the big negatives of incest is the higher probability of birth defects for a child. But what if a brother and sister have strong feelings for each other and they sleep together just once? Why is that considered wrong? The only reason I can think of is that it's been considered wrong for so long. I don't believe it's that uncommon with other animals. It seems to me that the only thing hindering it is that it's a taboo in society.
PS: I'm in no way condoning incest. I'm just curious.