I don't see anything morally wrong with incestual love. Love is Love no matter what shape it deigns to take.
However, the idea of children must be considered. If the couple wants to have children, that is. If they don't, then I fail to see the problem.
If they do want to have children and the child is likely to be severely affected as a result, I do not think it is acceptible. Getting together knowing that you will be bringing a child into a horrible life is not acceptible whether incestual or no, however.
But, if they just want to get together because they properly love each other, then all the best to the theoretical couple.