I don’t think it should be, assuming the relationships are mutual. My biggest concern is that a parent/relation could spend a child’s life bullying them/psychologically manipulating them in order to marry them.
Like ItsWhatEyeKnow says, genetics is also a concern. I view that as more secondary, however, as the likelihood of a single, or even a few, incestual pairings resulting in enormous genetic problems is pretty minor. I don’t feel that there will be enough family lines that are continually incestual that it will be a huge problem (although it can certainly happen, particularly when considering the psychological aspects of such an environment).