Before anyone accuses me of it, I really don't at all feel like this about my brother, dad, cousin or anyone...
But if two people, who are equals to each other (so one is not in a position of power over the other, like a parent, or grandparent etc.), and who are old enough to take responsibility for their actions want to have a relationship, I think that's really up to them, I don't think they should be treated as criminals.
Other taboos are starting to become more accepted, for example, I can't imagine ever wanting to be in a three-way relationship, and I can see that a lot of people could be hurt by that kind of thing, but it's not illegal, and if people want to open themselves up to that kinda thing it's really their choice.
The children issue is difficult (although that programme on channel 5 last week had a genetic brother and sister with 2 healthy children), both with physical problems and psychological issues, but the psychological issue at least, happens to thousands of children with all sorts of unusual parental set-ups. Also, parents who are known to have some sort of genetic disorder that will be passed onto their children are allowed to have children still, why should they be allowed that right, when relatives who are only risking a genetic problem rather than knowing it as a certainty aren't?