I personally don't have an issue with such an arrangement, but I don't think marriage would be an appropriate term for it since they would already have many of the rights that come with marriage and it is quite possible that they could change their minds if someone else came along, a lot more likely than people in marriages, as it is supposed to be now, where the vast majority are pledging this. A different contract that covered something like this would be more appropriate. Or being able to present this situation could work too.
As a side note, I have always had the position that all first cousins and further out family relations should be allowed to marry. It's not a bad idea to include genetic counseling in this, like many states that allow it do, but it should be completely legal.