This is very interesting morally and ethically.
We do have a long-standing instinctive feeling that incest is not right - maybe it is because genetically succesfull societies have been ones that have grown outwards, not inwards, genetically.
But in terms of a relationship between consenting and equal adult individuals, I find it hard to say it should be illegal. A symptom of emotional dysfunction, maybe (and the story of this couple may hint that way)but without the question of children, does it harm anyone else?
The children issue is far more complex.
There is a theory that as a species we are no longer evolving. That we adapt our environment, rather than evolve in response to it. Maybe we don't need to relay on genetic diversity to survive as we have in former ages. It isn't ilegal for two carriers of genetic conditions to have a child - though most seem to seek counselling to avoid passing on the condition - but i do find it hard to agree that siblings should even contemplate having children.
I think it's outrageous to suggest that there is a correlation with disabled people having children where there is no genetically transmittable cause. And in any case, many disabled parents do end up having thier children taken away because someone considers them not able to cae for them.
The needs of a child should always be paramount.....