the reasoning behind these laws seem to stem from the fact that it is morally wrong, but morality can be a sliding scale, morality changes from one situation to the next, I personally on a morality level don't see a problem with it, they are living their lives like they want, who am I to stop somebody from reaching their dreams or goals, just like I am living my life right now, if they are happy doing that then god speed for them. In fact, if brother and sister were incredibly happy with this arrangement then I am envious because so few of us truly are able to say that we are happy with all aspects of our lives. The truly murky terms comes to laws, since it is not a recognized marriage, then the couple will not be afforded the same benefits as a normal couple but we as humans wish to achieve happiness and if this is how they want to achieve it, go for it.
Scientifically, when a bro and sis have a child, there could be complications at the DNA level, technically the child should receive half the genes from the bro and half from the sis, but because both bro/sis got genes from an original source, there could be two sets of genes and depending on how the genes are spread a host of problems could occur. So from a science point of view I dont believe its a good idea, considering that you are not really propagating your genes since they are continuously drawing from a general source.
Would I personally have an incest relationship, no, there is no point in trying to pursue one, its just alot easier to have a familial bond and relationship as a brother/sister rather than lovers.