I don't think its appropriate. Sure, siblings can develop a special bond between them, but it does not mean that this merits a romantic/intimate relationship. Many people tend to confuse sibling love with romantic love and many are prone to becoming a couple.
On the matter of breeding, I'm sure that there are many couples outside of family ties that didn't plan on children when they were physical. The risk is always there, and if siblings engage in intercourse, there's always a possibility that they will conceive a child. If siblings are having procreation sex, then it's definitely an issue.
As far as what "society" says, society doesn't need to dictate what is right or wrong for us. People kill, lie, cheat, and steal on a nearly daily basis, and we are fully aware that's not right. Yet people continue to do it. People who engage in incest are (I believe) aware of what they are getting into, and that it is wrong on many levels.
Bottom line, people will do what they do and find some way to justify it. We are all guilty of our own actions and will keep our beliefs dear to ourselves.