why is incest wrong?

I was asked this question by one of my friends. As put off as I was, I was also left thinking: why exactly? Obviously, if two biologically related people decide to reproduce, chances are that their children will have some kind of physical deformity or mental disability. However, why is the case of adopted siblings, or two sisters? why are our brains wired to be repulsed at the mere thought of a relationship between siblings? And I would like to state that I 1000% disagree with incest in any, any, context.