There are also genetic reasons. It's much easier to avoid sibling relationships when both siblings have the fathers surname. Believe it or not, cases of half-siblings having relationships do happen. Think about it - both are likely to be of similar age and grow up not to far apart from one another. In cases where the father is not on the scene for either, chances are if the meet, and its not entirely unlikely, they will not suspect that they may be siblings. And it may not just be siblings, we could be talking about cousins not knowing they are cousins, which is even more likely if both families live in the same area. Not as uncommon as you think and is beginning to become a problem in poorer parts of some cities worldwide.