In general, not the state's business. The question of children is a very interesting one though. If you think on account of potential genetic problems (which by the way are still very unlikely with cousins or further, though several generations of inbreeding will increase the likelihood IIRC) we shouldn't allow children to be conceived in these circumstances, would you also ban people with inheritable genetic disorders from reproducing? Would you force the abortion of fetuses that will end up as severely disabled babies? I would be inclined to say no in these cases and thus I can't reasonably see how the incest case is different in this regard.
One thing to note though is that there are obvious questions of power imbalances that need to be brought up. I'm extremely socially liberal, but I have to take issue with the constant throwing around of the term "consenting adults", as if the age of 18 bestows magical powers of self-responsibility as soon as one attains it, as if it has a fundamental moral basis rather than a culturally dependent one. Imagine an 18 year old still lives with their parents and is financially dependent on them. Can you say that this 18 year old having sex with a parent is morally justifiable because it's simply a case of "consenting adults"? I doubt it.
So in general I would advocate no prison time or fine, but I recognise that this is not so black and white as "consenting adults, state shouldn't care".